SUMMARYAs well as the known sporidesmolides I, I1 and 111, Pithomyces chartarum produces sporidesmolides or similar compounds which contain residues of erythro-isoleucine and a-hydroxyisocaproic acid. When erythro-L-isoleucine was added to the growth medium, synthesis of sporidesmolides I and I11 was inhibited and synthesis of isoleucine-containing sporidesmolides, mainly sporidesmolide 11, was promoted. Threo-L-isoleucine in the medium was poorly utilized, but its presence resulted in production of a very complex mixture of sporidesmolides containing a higher proportion of isoleucine residues than the control. Epimerization of erythro-D-isoleucine occurred in the medium. Neither epimer was well utilized, and this isomer had little effect on sporidesmolide production or composition ; it promoted some increase in the synthesis of isoleucine-containing sporidesmolides. Threo-Disoleucine had effects qualitatively similar to but less pronounced than those of erythro-L-isoleucine, probably because of extracellular epimerization. The use of epimeric pairs of amino acids as tools for investigating biochemical processes involving inversion of configuration at an amino acid a-carbon atom is suggested.