Strains of E. coli from the faeces of patients with active ulcerative colitis and with colitis in remission were examined for haemolysin and necrotoxin production. Cultures from 63 patients with active colitis grew haemolytic E. coli in 23 (37%) as compared with 24 (21%) from 115 patients whose disease was in remission (p < 0.05). The corresponding proportions for necrotoxin-producing strains were 22% and 12%. Of 35 patients investigated both in relapse and remission of the colitis, 14 changed their carriage of haemolytic E. coli. Thirteen of these carried haemolytic organisms in relapse but not in remission, and one carried haemolytic organisms in remission but not in relapse (p < 0.01). Strains of E. coli were also examined from specimens of faeces obtained at weekly intervals for 28 weeks from 19 patients. The acquisition of haemolytic or necrotoxic E. coli strains tended to follow rather than precede the onset of the attack in the four patients who developed a relapse of the disease during this period. Among these 19 patients haemolytic and necrotoxic strains were found most commonly when blood was regularly present in the faeces. These facts suggest that conditions in the bowel during a relapse of colitis tend to favour the presence of haemolysin- and necrotoxin-producing organisms, rather than that these organisms cause the relapse. Of 50 strains of E. coli from patients treated with sulphasalazine tested for sensitivity to sulphapyridine 49 were resistant. There was no clear relation between the activity of the colitis and the presence of any of the 27 serotypes of E. coli examined.