The acetyl conjugation of T-2 toxin and its derivatives, the 12,13-epoxytrichothecene mycotoxins, was studied by using mycelia of trichothecene-producing strains of Fusarium graminearum, F. nivale, Calonectria nivalis, and F. sporotrichoides. T-2 toxin was efficiently converted into acetyl T-2 toxin by all strains except a T-2 toxin-producing strain of F. sporotrichoides, which hydrolyzed the substrate to HT-2 toxin and neosolaniol. HT-2 toxin was conjugated to 3-acetyl HT-2 toxin as an only product by mycelia of F. graminearum and C. nivalis, but was also resistant to conjugation by both F. nivale and F. sporotrichioides. Neosolaniol was also biotransformed selectively into 3-acetyl neosolaniol by F. graminearum. However, 3-acetyl HT-2 toxin was not acetylated by any of the strains under the conditions employed, but was hydrolyzed to HT-2 toxin by F. graminearum and F. nivale. This is the first report on the biological 3a-0-acetyl conjugation of T-2 toxin and its derivatives. T-2 toxin, 4,8,15-diacetoxy-8a-(3-methylbutyloxy)-3a-hydroxy-12,13-epoxytrichothec-9ene, is one of the most important trichothecene mycotoxins occurring naturally in agricultural products and associated with serious field toxicoses of humans and animals (5, 6). The metabolic fates of this toxin have been studied in fungi (12-14), livers (3, 9, 10), rats and mice (11), and chickens (T. Yoshizawa, S. P. Swanson, and C. J. Mirocha, submitted for publication). Up to