Twenty-one strains of Verocytotoxin-producing Escherichia coli (VTEC) that hybridized with DNA probe CVD419 were examined for the ability to produce haemolysin. With solid media, all strains produced most haemolysin when grown in blood agar tubes and least when grown on blood agar plates incubated in air. Haemolysin production was increased considerably by incubating blood agar plates in an atmosphere comprising 8% carbon dioxide, 40% hydrogen and 52% nitrogen at 37 degrees C for 16 h, followed by 6 h at 21 degrees C in air. Haemolysin production was also increased when strains were grown on L-agar containing the iron chelator ethylenediamine-di(o-hydroxyphenylacetic acid) prior to subculture on blood agar. Intracellular haemolysin was detected in five out of the 21 strains of E. coli grown on L-agar in the atmosphere described above, but haemolysin was not detected in L-broth culture supernatants. The haemolysins lysed guinea pig, mouse and ferret erythrocytes, but not human, rabbit, rat, turkey or chicken erythrocytes. Also, the addition of calcium ions to culture media was not required for haemolytic activity. It was concluded that haemolysins produced by VTEC appear to be quite distinct from E. coli alpha-haemolysin and resemble a form of beta-haemolysin.