A cytotoxic component(s) was detected in culture filtrates of Klebsiella oxvtoca isolated from patients with antibiotic-associated hemorrhagic colitis. Eleven of 12 isolates exhibited cytotoxicity on HEp-2 cells. The cytotoxic activity of K. oxvtoca strain MH43-1 was stable for the treatment of 60 C for 30 min, but inactivated by the treatment of 100 C for 15 min.This cytotoxicity was not destroyed by the treatment with trypsin or pronase, and the component was filtrable through a membrane filter which cut off molecular weight 5,000.