Shigella dysenteriae produced a toxin that caused morphological changes in Chinese hamster ovary cells which was different from the neurotoxin (Y. Takeda, K. Okamoto, and T. Miwatani, Infect. Immun. 18:546--548, 1977). It was found that the toxin was produced not only by S. dysenteriae but also by S. flexneri, S. boydii, and S. sonnei. The production of the toxin was stimulated by the addition of 1 microgram of mitomycin C per ml to the culture in the early logarithmic growth phase.