Cell culture is a fundamental tool in proteomics where mammalian cells are 
cultured in vitro using a growth medium often supplemented with 5-15% FBS. 
Contamination by bovine proteins is difficult to avoid because of adherence to 
the plastic vessel and the cultured cells. We have generated peptides from 
bovine serum using four sample preparation methods and analyzed the peptides by 
high mass accuracy LC-MS/MS. Distinguishing between bovine and human peptides is 
difficult because of a considerable overlap of identical tryptic peptide 
sequences. Pitfalls in interpretation, different database search strategies to 
minimize erroneous identifications and an augmented contaminant database are 
presented.