Microbial inocula from rumen fluid, soil, and contents of the large intestines of chickens (CLIC) and of swine (SLIC) were tested for their ability to transform deoxynivalenol (vomitoxin) in vitro. Microorganisms in (CLIC) completely transformed pure vomitoxin, and this activity was retained through six serial subcultures. No alteration of the toxin by incubation with SLIC was detected, whereas 35% of the vomitoxin was metabolized in the original culture of rumen fluid and 50% was metabolized by the soil sample, though metabolism was decreased in subsequent subcultures of either sample. A single metabolite was isolated and identified as deepoxy vomitoxin. The increase in concentration of deepoxy vomitoxin in the culture medium corresponded with the decrease in vomitoxin concentration. The vomitoxin transformation rate was not affected by either the ratio of CLIC to vomitoxin (5 to 0.2 g of CLIC per mg of vomitoxin) or the initial concentration of vomitoxin (14 to 1,400 ppm) in the medium. Biotransformation of vomitoxin was completely inhibited when the pH in the medium was lowered to 5.20. Sodium azide at a 0.1% (wt/vol) concentration in the medium blocked the transformation of vomitoxin, suggesting that the deepoxidation of vomitoxin is an energy-dependent process. About 50% of the vomitoxin in moldy corn in culture medium was transformed by microorganisms from CLIC. The vomitoxin transformation rate in moldy corn was not affected when the concentration of CLIC changed from 0.2 to 0.8 g/ml of medium. Vomitoxin in the moldy corn was not transformed when CLIC were added to corn without culture medium.(ABSTRACT TRUNCATED AT 250 WORDS)