August 2026 · 6 min read
Xeno-free. Serum-free. Animal component-free. Chemically defined. These four terms appear in almost every cell therapy protocol — and they are not synonyms. Using the wrong one in a regulatory document or manufacturing protocol can cause confusion, delays, and compliance questions. Here is exactly what each means.
The four terms — definitions
Serum-free
Definition: No serum (FBS, human serum, or any other whole serum) is present in the medium formulation.
What it does NOT exclude: Other animal-derived components — BSA, bovine transferrin, bovine insulin, trypsin, bovine pituitary extract, animal-derived growth factors. A medium can be serum-free while containing multiple bovine proteins.
Example: DMEM + 4 mg/mL BSA + 10 µg/mL bovine transferrin + 10 µg/mL bovine insulin = serum-free but NOT animal component-free.
Xeno-free
Definition: No components derived from a species other than the species being cultured. For human cell culture — no bovine, porcine, murine, or other non-human animal-derived components. Human-derived materials (human albumin, human transferrin, human platelet lysate) are acceptable.
What it does NOT exclude: Human-derived biological materials. hPL, human AB serum, recombinant human albumin (rHSA), recombinant human transferrin, and human plasma-derived components are all xeno-free for human cell culture.
Example: DMEM + 5% human platelet lysate + rHSA + OsrhTF = xeno-free but NOT serum-free (hPL is serum-equivalent) and NOT animal component-free (hPL is a biological material).
Animal Component-Free (ACF)
Definition: No components derived from any animal source — including human donors. All components are either synthetic or produced by microbial/plant recombinant expression.
What it excludes: FBS, human serum, hPL, BSA, bovine transferrin, bovine insulin — and also rHSA if produced in a mammalian expression system. Plant-expressed rHSA (e.g. OsrhHSA from rice) and microbially-expressed components are ACF.
Example: Chemically defined basal medium + OsrhHSA (rice-expressed) + OsrhTF (rice-expressed) + recombinant human insulin (E. coli-expressed) + recombinant cytokines = ACF.
Chemically Defined
Definition: Every component is a purified, identified chemical entity with a known molecular structure and concentration. No undefined biological mixtures.
What it excludes: All whole sera (FBS, human serum), hPL, and any undefined biological extract. Recombinant proteins are chemically defined if they are fully characterised.
What it includes: Amino acids, vitamins, inorganic salts, recombinant cytokines, recombinant growth factors, recombinant albumin, synthetic lipids — all at defined concentrations.
The relationship between the terms
| Medium type | Serum-free? | Xeno-free? | Animal component-free? | Chemically defined? |
|---|---|---|---|---|
| DMEM + 10% FBS | No | No | No | No |
| DMEM + 5% human AB serum | No | Yes (for human cells) | No (human-derived) | No |
| DMEM + 5% hPL + rHSA | No (hPL is serum-equivalent) | Yes | No | No |
| DMEM + BSA + bovine transferrin | Yes | No | No | No |
| DMEM + rHSA + OsrhTF + recombinant insulin | Yes | Yes | Yes | Yes (if all components defined) |
| mTeSR1 (StemCell Technologies) | Yes | No (contains BSA) | No | No |
| TeSR-E8 (StemCell Technologies) | Yes | No (contains BSA) | No | Partially |
Which term does your application actually require?
ATMP manufacturing (MSC, CAR-T, NK cells)
Required: Xeno-free at minimum. EMA’s Advanced Therapy Guidelines explicitly state that xeno-free conditions should be used for clinical-grade human cell manufacturing to eliminate the risk of xenogenic immunogenic reactions in patients. Animal component-free is preferred for the highest regulatory confidence. Chemically defined is the ultimate goal but not yet universally achievable for all cell types.
Research in human cell biology
Serum-free is usually sufficient. Most researchers describing their medium as “serum-free” mean they are not using FBS — they may still use BSA or bovine transferrin without specific concern. This is scientifically acceptable for non-clinical research but should be accurately described in publications.
GMP manufacturing — IND/BLA submissions
Animal component-free is increasingly expected. Regulatory reviewers for cell therapy INDs now routinely ask about all animal-derived raw materials — not just serum. A manufacturing process using bovine-derived BSA or transferrin requires the same TSE/BSE documentation as FBS. Using recombinant human alternatives (rHSA, OsrhTF) eliminates this documentation burden entirely.
Process development and screening
Any level may be appropriate. Start with serum-containing conditions for feasibility; transition toward xeno-free or ACF as the process matures toward clinical application. Changing the medium formulation at any stage requires re-validation — plan transitions early.
SeamlessBio products for each category
| Category | Product | Available from SeamlessBio |
|---|---|---|
| Xeno-free serum equivalent | Human AB Serum, Human Platelet Lysate (hPL) | Human Serum Portfolio |
| Xeno-free + ACF albumin | rHSA (OsrhHSA, rice-expressed) | rHSA |
| ACF iron delivery | OsrhTF (recombinant human transferrin, rice-expressed) | OsrhTF |
| Serum-free carrier (non-human) | BSA Standard, Low Endotoxin, Fatty Acid Free | BSA Portfolio |
Further reading:
English 
Deutsch