A serological assay was developed to assess the outcome of the treatment of intestinal schistosomiasis with praziquantel, in patients with Schistosoma mansoni infection. Each of 33 patients (seven found to be excreting S. mansoni eggs and 26 egg-negatives found seropositive for antibodies against antigens from S. mansoni eggs) had two to five serum specimens assayed, the sera being collected at the time of diagnosis and at least once after praziquantel treatment. The sera were tested in ELISA against three antigen preparations: the unfractionated soluble egg antigens of S. margrebowiei and S. mansoni (SmSEA) and a cationic antigen fraction (CEF6) purified from the SmSEA. The dynamics of the post-treatment antibody levels were variable. In a minority of the patients, antibody levels declined relatively rapidly (within 5-12 months), to ELISA negativity, with the levels of the anti-CEF6 antibodies declining more rapidly than those of the anti-SmSEA antibodies. In the remaining patients, however, the levels of these specific antibodies declined only slowly or not at all over a 2- to 3-year period. The post-treatment monitoring of the levels of anti-schistosome-egg antibodies, particularly those of anti-CEF6 antibodies, may help to distinguish the treatments that result in parasitological cure from those that are only partially successful.