By using paraffin sections of adult schistosomes fixed in Rossman's fixative, specific human IgM and IgG antibodies to a polysaccharide present in the epithelial cells of the schistosome were measured by using indirect immunofluorescent techniques. Of the 49 patients, mostly infected with S. mansoni but a few infected with S. haematobium or S. japonicum, all had antibody present at a 1:8 dilution of serum. Specific IgM antibody was more sensitive than IgG, yielding 100% and 86% positivity respectively. The false positive rate was 3% in a panel of sera obtained from patients most of whom were infected with other parasites. Antibodies were detected by the 3rd week in experimentally infected animals. Unisexual infections also induced antibody production. As a diagnostic test, the measurement of antibody to the polysaccharide is an easily performed reliable test with high sensitivity and specificity.