In relation to the mechanism of fatty acidsynthetaseinhibition by cerulenin (1) the reaction between this antibiotic and thiols were studied. The structures of the propanethiol adduct 2 and of the cysteine methyl ester adduct 3 were determined to be respective (2S, 3R) 2 and of the cysteine methyl ester adduct 3 were determined to be in their 4-oxo-amide moiety.