Isoagglutinins were studied in patients with acute schistosomiasis and visceral larva migrans (VLM). Anti-A and anti-B titers were found to be significantly higher in both groups of patients than in the controls. An increased incidence of low molecular weight agglutinins to A cells was found in both patient groups and to B cells in the VLM group. Purified IgG fractions were prepared from the sera of 13 patients with VLM and 12 of these were found to contain isoagglutinins, some in high titer. Two high titered fractions were tested with a panel of cord blood group A erythrocytes. The infant red blood cells were readily agglutinated but in slightly lower titer (one to four tubes) than adult A1 cells.