A cytotoxin (CytK) has been isolated from a Bacillus cereus strain that caused a severe food poisoning outbreak killing three people. A protein of 34 kDa was highly cytotoxic, and the addition of other secreted proteins gave no synergistic effect. CytK was also necrotic and haemolytic. No known B. cereus enterotoxins were produced by this strain. A DNA sequence from 1.8 kb upstream to 0.2 kb downstream of the toxin gene was sequenced. The deduced amino acid sequence of the toxin showed similarity to Staphylococcus aureus leucocidins, gamma-haemolysin and alpha-haemolysin, Clostridium perfringens beta-toxin and B. cereus haemolysin II, all belonging to a family of beta-barrel channel-forming toxins. There was no sequence similarity between CytK and enterotoxins of B. cereus. The upstream sequence contained a partial sequence of a putative histidine kinase gene. A recognition site for PlcR, which regulates the transcription of enterotoxins HBL and Nhe of B. cereus, was found in the promoter region of the toxin. This new cytotoxin may be responsible for a disease that is similar to, although not as severe as, the necrotic enteritis caused by the beta-toxin of C. perfringens type C.