The anamorphic basidomycetous yeast Cryptococcus humicolus was shown by hydride generation-gas chromatog.-at. absorption spectrometry to methylate inorg. antimony compds. to mono-, di-, and trimethylantimony species under oxic growth conditions. Methylantimony levels were pos. correlated with initial substrate concns. up to 300 mg Sb l-1as potassium antimony tartrate (K-Sb-tartrate). Increasing concns. of K-Sb-tartrate increased the ratio of di- to trimethylantimony species, indicating that methylation of dimethylantimony was rate limiting. Antimony methylation capability in C. humicolus was developed after the exponential growth phase and was dependent upon protein synthesis in the early stationary phase. Inclusion of inorg. arsenic (III) or (V) species alongside antimony in culture incubations enhanced antimony methylation. Pre-incubation of cells with inorg. arsenic (III) further induced antimony methylation capability, whereas pre-incubation with inorg. antimony (III) did not. Exposure of cells to inorg. arsenic-either through pre-incubation or provision during cultivation-influenced the antimony speciation; involatile trimethylantimony species was the sole methylated antimony species detected, i.e. mono- and dimethylantimony species were not detected. Competitive inhibition of antimony methylation was obsd. at high arsenic loadings. These data indicate that antimony methylation is a fortuitous process, catalyzed at least in part by enzymes responsible for arsenic methylation.