The toxic parasporal protein of Bacillus thuringiensis var. sotto Ishiwata solubilized in vivo in gut juice, and in vitro under alkaline conditions, has been studied by serological methods. Antibodies elicited by alkali-dissolved protein from the crystals of Bacillus thuringiensis var. thuringiensis Berliner, Bacillus entomocidus var. entomocidus Heimpel and Angus, and B. thuringiensis var. sotto, inactivate the sotto protein in vitro and in the gut of larvae of the silkworm, Bombyx mori (linnaeus). Injections of toxin-specific antibody into the hemocoel do not protect larvae against the toxic effects of ingested crystals.