Antibacterial activity of pure preparations of mycotoxins was studied against strains of B. thuringiensis. Most of the fungal metabolites used provoked growth bacterial inhibition similar to lethal effect of antibiotics and enlargement of cell volume. This cell abnormality induction resembled those obtained with mitomycin C. The bioassay of fungal toxins with B. thuringiensis can be recommended as a sensitive technique to supplement chemical characterization, by using specific cell lines for such mycotoxins and specific inactivators such as coumarin, for aflatoxin or sulfhydryl compounds for penicillic acid and patulin.