A standardized microtest plate enzyme-linked immunosorbent assay was developed using the microsomal fraction of adult worms of Schistosoma mansoni (MAMA) as antigen. The standard reference serum pool was prepared from acutely and chronically infected rhesus monkeys and was shown to be appropriate as a standard for measuring the levels of reactivity of the unknowns. The standard serum pool was arbitrarily designated as having 100 activity units per µl. The levels of reactivity of the unknowns were expressed as activity units per µl. Serum specimens were obtained from 190 patients infected with S. mansoni in the Caribbean, South America, and Africa. Serum was obtained from small numbers of patients infected with S. haematobium, S. japonicum, or S. mekongi. Controls were 136 patients with other helminthic infections, 142 patients with protozoal or other diseases with liver involvement, and 81 healthy serum donors. The J index (or predictability) of the assay was calculated to determine the significant level of reactivity. The assay has a predictability of 95% for both patients with S. mansoni infections and those with other infections. The sensitivity of the assay for S. mansoni infections was 96%, and the specificity (in terms of cross-reactions with infections with other parasite genera or with other liver diseases) was 99%. The heterologous Schistosoma species showed a markedly lower level of reactivity, with an overall sensitivity of 55%. This is in accord with the species-specificity previously recognized in MAMA, and emphasizes the need for standard reference pools of human sera prepared from patients infected with single species of each of the Schistosoma. Use of these pools in assays with antigens of the respective schistosome species would allow optimum serologic evaluation.