Bacillus cereus strain UW85 produces an antibiotic, designated zwittermicin A, that is associated with the ability of UW85 to suppress damping-off disease of alfalfa (Medicago sativa) caused by the oomycete pathogen, Phytophthora medicaginis, in a laboratory bioassay. We have identified certain culture conditions that promote or suppress zwittermicin A accumulation by UW85. Maximum accumulation was detected in supernatants of trypticase soy broth cultures after sporulation, which is when cultures of UW85 provide the greatest suppression of damping-off on alfalfa. Inorganic amendments to trypticase soy broth cultures had the following effects on zwittermicin A accumulation and disease suppression : phosphate (50 mM or more) reduced zwittermicin A accumulation and disease suppression; ferric iron (0.25-1.0 mM) enhanced zwittermicin A accumulation and disease suppression; micronutrients (manganese, boron, copper, molybdenum, zinc) had no effect on zwittermicin A accumulation or disease suppression. Cultures of UW85 grown in chemically defined minimal medium supplemented with casein hydrolysate or grown in defined medium containing the minimal requirements for growth supplemented with five amino acids (Gin, Arg, Met, Phe, Ile) accumulated zwittermicin A. In minimal medium, alfalfa seed exudate inhibited growth of UW85, whereas alfalfa sprout exudate enhanced zwittermicin A accumulation by 40%. These data indicate that the accumulation of zwittermicin A can be modulated by specific nutrients, inorganic compounds, and plant-derived factors. These results will facilitate the improvement of large-scale purification of zwittermicin A, suggest appropriate conditions under which to conduct further genetic and biochemical analyses, and further substantiate the association between antibiotic accumulation and disease suppression by UW85.