Potent inhibition of cell proliferation was found for commercial preparations of bovine alpha-lactalbumin on cultured intestinal cell line IEC-6 albeit lot-dependent. The inhibition was irreversible and a single exposure to the culture medium containing alpha-lactalbumin of an active lot for a period as short as 30 min was enough to provoke cell death, possibly through apoptosis. The oligomer fraction from size exclusion chromatography was significantly robust, while the monomer fraction remained totally inert, in inducing cell death. Incubation at 37 degrees C for 5 d with 30% trifluoroethanol in acetate, pH 5.5, in a slowly rotating test tube rendered the monomer fraction cytotoxic. Again, the resulting inhibitory activity was found in the oligomer fraction from size exclusion chromatography, with emergence of subtle peaks at 22- and 30-kDa. Furthermore, the occurrence of SDS-stable 30-kDa as well as 20-kDa bands on electrophoresis was a common feature for alpha-lactalbumin with the activity inducing cell death. Thus, a certain dimeric state can be implicated in the cytotoxicity of bovine alpha-lactalbumin.