The growth of the estrogen responsive human breast cancer cell line, MCF-7, is inhibited by high serum concentrations, and this growth inhibition can be abolished by estradiol (E2). To investigate this inhibitory phenomenon further, we decided to purify the inhibitory factor from newborn calf serum (NCS). After the use of various fractionation methods, we found that inhibitory activity in NCS was exclusively expressed by albumin containing fractions. The inhibitory potential of several commercial bovine serum albumin (BSA) preparations and one human serum albumin preparation were analysed. They all exerted inhibitory activity comparable to that of NCS, and BSA inhibited MCF-7 cell proliferation in a concentration-dependent manner similar to that of NCS. Albumin itself or a contaminating factor in the albumin preparations seemed to be responsible for the growth inhibition. It could be excluded that the growth inhibitor TGF-beta, known to be present in serum, was the factor which inhibited MCF-7 cell proliferation. We separated contaminating proteins from albumin by gel filtration of a BSA preparation, revealing that neither low mol.wt nor high mol.wt proteins in the preparation exerted any significant growth inhibitory activity. NCS and BSA affected the secretion of specific proteins from MCF-7 cells similarly, when grown with or without E2. In conclusion, we assume that albumin is the factor in serum exerting a growth inhibition which can be reversed by E2. Our results indicate that albumin may affect cell proliferation by modulating the activities of autocrine growth regulatory factors.