Crude liver-microsome fractions (9000 g supernatant) prepared from the livers of chick, duckling, guinea-pig and mouse metabolized G 1 as rapidly as B 1, a result previously reported. Unidentified metabolites of aflatoxin B 1 and G 1 (designated BX and GX) have spectral characteristics similar to the aflatoxin hemiacetals B 2a and G 2a. In particular, methanol extracts of liver incubations showed peak absorption at about 400 nm at pH 6·5 or more and the absorbance was intensified when strong ammonia solution was added. Chemically prepared B 2a and G 2a was found to be unstable in dilute phosphate-buffered protein solution at pH 7·4, being first bound to protein and subsequently degraded to form yellow products. The latter degradation was inhibited by adding cysteine (5 mM) and when this was incorporated in a standard incubation mixture, about 50% of the aflatoxin B 1 metabolized by excess chick-liver fractions was spectrophotometrically identified as its hemiacetal. This biotransformation was plausible because of the simplicity of the proposed enzymic 2-hydroxylation of aflatoxin, the associated loss of hepatotoxicity and the spectrophotometric evidence. The instability of aflatoxin hemiacetals (B 2a and G 2a) in protein solution at physiological pH may explain their low acute toxicity.