A survey was made to detect microorganisms useful for assaying butenolide [4-acetamido-4-hydroxy-2-butenoic acid gamma-lactone] and T-2 toxin [4beta, 15-diacetoxy-8alpha-(3-methylbutyryloxy)-12,13-epoxytricothec -9-en-3alpha-ol]. These mycotoxins produced by strains of Fusarium tricinctum have been implicated in mycotoxicosis of livestock. Although butenolide proved to be a very weak antibiotic, assay discs containing 100 mug of this toxin inhibited Sprillum serpens NRRL B-2052, Vibrio tyrogenus NRRL B-1033, and Xanthomonas campestris NRRL B-1459. T-2 toxin had no effect on 54 bacterial strains but inhibited 6 of 11 fungi. Growth of Rhodotorula rubra NRRL Y-7222 and Penicillium digitatum NRRL 1202 was retarded by assay discs containing 4 mug of T-2 toxin. Solutions with less than 1 mug of T-2 per ml toxin were readily detected by a pea seed germination test. Germination was reduced more than 50% when seeds imbibed solutions of 0.5 mug of T-2 toxin per ml. Butenolide had no effect on pea seed germination at concentrations as high as 200 mug/ml.