Eighty-eight of 1529 (5.7 per cent) Escherichia coli isolates from diarrhoeic and clinically normal calves in Scotland and northern England were found to possess the K99 pilus antigen (K99+). There was complete correlation between possession of K99 antigen, heat stable enterotoxin production and ability to dilate intestinal loops. The diagnosis of calf enterotoxigenic E coli infections may therefore be based on the detection of K99 antigen alone. Enterotoxigenic E coli was isolated from 23 of 306 (7.5 per cent) diarrhoeic calves from eight of 70 (11.4 per cent) farms and was not isolated from clinically normal calves. Infected calves were between one and three days old. A survey by an enzyme-linked immunosorbent assay found 3.0 per cent and 3.9 per cent of sera from calves and cows respectively to contain antibodies to K99 antigen. The prevalence of other enteropathogenic organisms in calf faeces is also discussed.