SUMMARY: Spores of Bacillus thuringiensis contain a toxin active against lepidopterous larvae. This toxin can be solubilized extraction with reagents which dissolve the protein crystal of B. thuringiensis. It is inactivated crystal-specific antiserum. Spores of Bacillus cereus contain a similar toxin although the specific activity is much lower than the spores of B. thuringiensis. The B. cereus toxin contains a single major polypeptide component. Toxic activity can be solubilized from spores of both species incubation with gut juices from Pieris brassicae.