Previous studies have demonstrated that cell populations that are cultured with heterologous animal products can acquire xenoantigens, potentially limiting their clinical utility because of immune responses. Embryonic stem cells (ESCs) are an attractive source of multiple potential cellular therapies and are typically derived and routinely cultured on murine embryonic fibroblast (MEF) feeder cell layers in commercially available serum replacement (SR) medium or fetal calf serum (FCS)-containing medium. Recently, we found that a strong antibody response was generated in human subjects after the second infusion of therapeutic cells cultured in FCS-containing medium. This response was specific for bovine apolipoprotein B-100 (apoB-100), which is the major protein component of low-density lipoproteins (LDL) and which targets its binding to abundant low-density lipoprotein receptors on the cell surface, from which it is internalized. Here, we have shown that ESCs cultured on MEFs in SR medium acquired bovine apoB-100 from MEFs and from the SR medium as well. Our findings also suggest that bovine LDL are used as critical nutrients for ESC propagation.