The insecticidal properties of delta-endotoxins from Bacillus thuringiensis (Bt) serotypes kurstaki and israelensis and crystal proteins of Bacillus sphaericus (Bs) serotype H5 have been used in insect control for decades. The availability of microbial toxins in biopesticides as well as in plants with incorporated protection has been increasing the concerns about biosafety. Acute toxicity to Danio rerio and cytotoxicity on mouse bone marrow cells and peripheral erythrocytes of Oreochromis niloticus were tested with Bt israelensis, Bt kurstaki and Bs H5 strains. The concentration and dose tested were 10(6) and 10(8) spores/ml, respectively. Neither lethality nor effects on mouse bone marrow were promoted by any strain. In necrosis-apoptosis study on peripheral erythrocytes of O. niloticus an increased frequency of necrotic cells caused by exposure to strains of B. thuringiensis was found. Exposure to B. sphaericus did not show cytotoxic effects in either tested system. None of the strains studied induced apoptosis in contrast with the chemical controls.