The binding of bovine (BSA) and human serum albumin (HSA) to the surface of human and mouse tumor cells in vitro was examined by flow cytometry using monoclonal antibodies against these albumins. Both isologous and heterologous types of albumin bound firmly on several lines of human and mouse tumor cells. The cell-bound albumin was removable by protease (actinase) treatment or by culture in albumin-free medium for more than 2 days, but not by simple washing. The amount of albumin capable of binding to the tumor cell surface differed among the 9 tumor cell lines tested. As determined with radioiodinated BSA, about 2.6 x 10(6) BSA molecules/cell could bind to MDA-MB-453 human mammary cancer cells, which exhibit high binding capacity as to BSA. A unique peptide having a molecular weight of 18 kDa was detected on gel electrophoresis in extracts of tumor cells or normal aortic endothelial cells which had been treated with radioiodinated, photocross-linker-labeled BSA or HSA, followed by UV-irradiation. This peptide was also immuno-precipitated with an anti-BSA or anti-HSA monoclonal antibody. With both methods, the yield of the peptide was decreased by previous addition of an excess amount of unlabeled BSA or HSA. These findings indicated that the 18-kDa peptide expressed on both normal aortic endothelial cells and tumor cells is a principal serum albumin-binding protein, and that this protein binds both BSA and HSA.