Sports from Bacillus thuringiensis serovars kurstaki and entomocidus synergized crystal protein toxicity for larvae of the Indianmeal moth (Plodia interpunctella). Preparations of spore-crystal mixtures of either serovar were more toxic for the larvae than either purified sports or crystals alone (based on dry weight). Spores lost 53% of their toxicity for the Indianmeal moth after 2 h of W-irradiation, but remained partially toxic (28%) even after 4 h of irradiation, Spore coat protein was toxic for the Indianmeal moth and was synergistic with B. thuringiensis serovar kurstaki HD-1 crystal protein. Enhanced toxicity of the combined sport-crystal preparation was attributed to a combination of crystal and spore coat protein, and included the effects of spore germination and resulting septicemia in the larval hemolymph. Ultraviolet irradiation of spores reduced the toxicity from septicemia but not the synergism caused by spore coat protein. The potencies of spore-crystal preparations must be carefully evaluated on the basis of contributions from all three factors.