Two anti-aflatoxin B1 antisera, antiserum 'O' and antiserum 'C', were raised against conjugates in which the bovine serum albumin protein carrier was coupled to aflatoxin B1 derivatives at carbon 1 or 8. Their action was investigated in Salmonella thyphimurium assays using a fortified 9000 x g supernatant (S9) of rat liver homogenate or using rat hepatocytes as the metabolic activation system. In all cases, a substantial reduction in mutagenicity was observed: the mutagenicity caused by 12.5 ng aflatoxin B1 was inhibited by 50% with 2 microliters of antiserum 'O' and with 12 microliters of antiserum 'C' when activation was catalysed by rat liver supernatant; the mutagenicity of 3.31 micrograms aflatoxin B1 was inhibited by 50% with 25 microliters of antiserum 'O' in experiments using hepatocytes as the metabolic activation system. Several lines of evidence indicate that the inhibition is caused by specific antibodies. The molar ratios of aflatoxin B2 or aflatoxin G2 to aflatoxin B1 required to reduce by 50% the effect of antisera on aflatoxin B1 mutagenicity were 2.9-76-fold higher than those required to reduce by 50% the tracer antibody binding in radioimmunoassay. Possible mechanisms by which anti-aflatoxin B1 antisera inhibit aflatoxin B1 mutagenicity are discussed.