We examined sera from 10 patients with hemolytic uremic syndrome (HUS) and 51 controls, with and without diarrhea, for antibodies to Escherichia coli lipopolysaccharides (LPS) O157, O111 and O26 using the indirect hemagglutination (IHA) assay. A significant rise (to a titer of > or = 2560) in IHA antibody to O157 LPS was detected in eight of the 10 HUS patients, to O111 in two patients, one of whom showed concomitantly an antibody rise to O157, but to O26 in no patients. The IHA titers fell rapidly after the acute phase of the illness. Of the control sera 15 (29.4%) non-specifically agglutinated uncoated sheep red blood cells (SRBC) at a titer of > or = 80, six (3.9%) at > or = 320 and the maximum was 640. In spite of the relatively low level of non-specific agglutination the IHA appeared to be a useful screening method to identify verotoxin-producing E. coli infections at the early stage of HUS because the titers were clearly higher than non-specific agglutination and the assay is easy to perform and gives results quickly. Artificial carriers are being considered for use in place of SRBC to diminish the non-specific hemagglutination.