The cytotoxic response of Bacillus thuringiensis δ-endotoxin with swelling like a balloon and bursting seemed to be universal on the cells taken from insects in virto, while cells developed from mammalian origin showed no morphological changes even at the ultrastructural level. δ-Endotoxin may not have other effects on mammalian cells, such as growth-inhibition of Yoshida ascites sarcoma cells and release-stimulation of UV-absorbing substances from mouse sarcoma 180 ascites cells into the medium.