Effects of aflatoxin B1 and three of its metabolites on cellular immune response were assessed with an assay based on inhibition of tritiated thymidine uptake by phytohemeagglutinin stimulated lymphocytes. In this in vitro system aflatoxin B1 and aflatoxin Q1 were strongly inhibitory (more than 50% inhibition) at concentrations of 10 mu/ml, whereas aflatoxicol and aflatoxin B2 alpha exhibited little inhibition at 10 micrograms/ml and only 45 to 50% inhibition at 25 micrograms/ml. Contrasts with single degrees of freedom and orthogonal polynomial analysis revealed that the pair of aflatoxin B1 and aflatoxin Q1 differed linearly and quadratically from the pair of aflatoxin B2 alpha and aflatoxicol, but within each pair there were no differences. Limited data with aflatoxin M1 revealed that it was slightly more active than aflatoxicol in the assay, but minimal replication prevented rigorous statistical testing. It may be theorized that the moderate to strong inhibition of blastogenesis by aflatoxin B1 and its metabolites could inhibit T lymphocyte functions, such as killer, helper, effector, or other immune processes, and thus compromise the immunological surveillance mechanism.