What’s Really Inside Fetal Bovine Serum?

For many researchers, reaching for a bottle of fetal bovine serum (FBS) is a daily habit performed without a second thought regarding its specific ingredients.

A new study led by researchers at Dalhousie University used high-resolution mass spectrometry to create the first comprehensive molecular map of FBS. Their findings reveal that while a core set of proteins exists, key growth factors and peptides vary significantly between commercial grades.

Addressing the reproducibility crisis in FBS

FBS is the backbone of modern biology. Scientists use it to grow cells for everything from cancer research to vaccine production. Despite its important role, FBS remains a black box in the laboratory, with researchers relying on it daily while rarely knowing its exact molecular makeup.

This lack of clarity is creating a reproducibility crisis in science. Experiments that work in one lab often fail in another simply because the serum batch changed.

“FBS is a widely used but poorly defined mammalian cell culture supplement, introducing substantial batch-to-batch and grade-specific variability that compromises experimental reproducibility,” said the authors of the latest study.

When the ingredients are unknown, the results are unpredictable, and this variability can do more than just stop cells from growing.

“The undefined and variable factors found in FBS may also affect gene expression patterns in cultured cells,” the authors added.

These hidden factors could change how a cell behaves, potentially leading scientists to the wrong conclusions.

Current research often focuses on small markers or basic nutrients; however, there is a massive gap in understanding the full protein landscape. No previous study has looked at the entire proteome alongside native peptides to see how commercial grades differ.

By using advanced mass spectrometry, the team aimed to perform the first simultaneous mapping of proteins and peptides across multiple serum grades.

Mapping serum grades using proteomics and peptidomics

The researchers used an integrative multiomics approach to compare low, medium, and high grades of commercial FBS using a technique called data-independent acquisition mass spectrometry.

“We combined proteomics and peptidomics to define the conserved molecular baseline and to quantify where grades diverge in ways that are likely to influence proliferation, adhesion, differentiation, and survival,” said the authors.

They used a process called ammonium sulfate precipitation and solid-phase preparation to clean the protein samples. To find the smaller protein fragments, or peptides, they used a filter with a 10 kDa cutoff.

“A conserved core of 979 proteins shared across all grades was enriched for extracellular matrix organization, immune modulation, and growth factor regulation, explaining FBS’s broad utility in standard cell culture,” the authors explained.

While the core was stable, the team found 88 proteins and over 200 peptides that varied significantly.

High-grade serum was packed with proteins such as fibronectin and vitronectin, which help cells adhere and grow. It also contained more growth-factor-binding proteins.

Whereas low-grade serum showed higher levels of proteins related to stress and detoxification.

The peptides showed even more variation than the whole proteins, suggesting that the way serum is processed and filtered during manufacturing is what truly determines its quality.

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Future implications for FBS selection and synthetic media

These findings could impact how we view serum selection; instead of guessing which bottle might work, scientists can now use data.

“This study addresses a persistent gap in cell culture practice, namely that FBS is used as a black box even though its molecular composition varies by product grade,” said the authors.

By knowing exactly what is inside, labs can ensure that their work stays consistent over years of research.

The study also hints at the possibility of new quality control tools. The high variability in the peptidome means that these small protein fragments could be used as biomarkers to monitor consistency during production.

Identifying the specific protein modules that help cells thrive could also help facilitate the move away from animal-derived serum toward more reliable, synthetic alternatives.

However, the researchers noted that their method for removing common proteins might have missed some very rare molecules.

Despite this, the work provides an important foundation: “Using mass spectrometry informed panels creates an evidence base for engineering defined media that retain the functional virtues of serum without its unpredictability,” the authors concluded.

Reference: Wijerathna-Yapa A, Sokolenko S. What is in fetal bovine serum used for cell culture? Integrated proteomics and peptidomics reveal grade-dependent molecular profiles. J Proteome Res. 2026:acs.jproteome.5c01097. doi: 10.1021/acs.jproteome.5c01097