The existence of a shared epitope between the hemocyanin of the marine mollusk Megathura crenulata, better known as the keyhole limpet, and schistosomula has been reported. This epitope has been shown to be a major immunogen in human infection. In this study, keyhole limpet hemocyanin (KLH) was used to measure antibodies recognizing the cross-reacting epitopes in sera from patients with acute and chronic schistosomiasis using an enzyme-linked immunosorbent assay (ELISA). Marked differences in IgG and IgM antibody response were noted between acutely and chronically infected patients at a reciprocal serum dilution of up to 2,560. The acute sera had a mean ± SD OD490 nm values for IgG and IgM of 1.0 ± 0.44 and 1.34 ± 0.6 compared to mean ± SD IgG and IgM absorbance for the chronic sera of 0.22 ± 0.10 and 0.22 ± 0.11 respectively. Setting our lowest positive limit at > 2 SD above the mean of the chronic sera, 28 of the 30 patients previously diagnosed as having acute schistosomiasis were correctly identified by their IgG and IgM response. Of 5 patients studied longitudinally, IgG persisted at the same levels 10–13 weeks after treatment. IgM levels, on the other hand, showed a tendency to decrease but remained above the established cut-off level. This study provides further evidence for the association of schistosomulum surface carbohydrate antibody with acute infection and demonstrates the ability of a simple non-competitive ELISA using microtiter plates coated with minute quantities of KLH to differentiate serologically between cases of acute and chronic schistosomiasis.