To test the value of serological tests as an adjunct to bacteriological methods and toxin testing in haemolytic uraemic syndrome (HUS), 60 patients with the disorder were examined for evidence of faecal Escherichia coli producing verocytotoxin (VTEC), particularly of serogroup O157. They were also tested for serum antibodies reacting with the lipopolysaccharide of E coli O157 by means of an enzyme-linked immunosorbent assay (ELISA) and immunoblotting; for faecal VTEC by means of DNA probes hybridising with the genes encoding verocytotoxins VT1 and VT2; and for "free" faecal VT. Strains of E coli serotype O157:H7 were isolated from 9 patients, and faecal VT2 was detected in 3 of them. Strains of E coli of serotypes 05:H-, O55:H10, O105ac:H18, and O163:H19 were isolated from 4 patients, but faecal VT was not detected. Faecal VT2 was present in 1 patient from whom VTEC were not isolated. Antibodies to the lipopolysaccharide of E coli O157 were detected in serum samples from 44 patients. The 9 patients with faecal O157:H7 all had high titres of these antibodies, but serum samples from 16 healthy control children were negative. Serological testing of patients with HUS for antibodies to the lipopolysaccharide of E coli O157 provides evidence of infection with E coli O157 when faecal bacteria or VT cannot be detected.