Human exposure to bovine serum albumin (BSA) is very common and occurs through dietary and medicinal routes. Although great effort has been made to reduce exposure to BSA in pharmaceuticals to eliminate the threat of bovine spongiform encephalopathy, less attention has been given to assessing the human immune response after exposure to BSA. A sensitive quantitative radioimmunoassay was therefore developed to measure anti-BSA IgG antibodies in healthy subjects and in cancer patients participating in a randomized, placebo controlled clinical trial where they were exposed to BSA as an intrathoracic surgical sealant during pneumonectomy. Anti-BSA antibodies were detected in 55% of 60 healthy blood donors and 51% of 83 patients before lung cancer resection. The median antibody levels were the same in both cohorts; 0.086 microg/mL (range 0.016-19.5 microg/mL) for health blood donors and 0.062 microg/mL (range 0.009-44 microg/mL) for cancer patients. Six months after exposure of the cancer patients to BSA, the percentage of patients with anti-BSA antibody rose to 96% and the median antibody level rose to 19 microg/mL (range 0.009-258 microg/mL). Placebo-treated cancer patients showed no significant increase in the percentage of patients with anti-BSA antibody (41%) or the median antibody level (0.047 microg/mL; range 0.008-1.58) over 6 months. Western blot analysis confirmed the presence of anti-BSA antibody. Elevated levels of anti-BSA antibody were not associated with any detectable clinical events in either the healthy blood donors or the cancer patients.