The purpose of the study was to determine the reliability of the circumoval precipitin reaction, a serologic test for the diagnosis ofSchistosoma mansoniinfections, and to find out whether it can be used to appraise the effect of chemotherapy in this disease. The circumoval precipitin reaction was performed in blood sera from 300 individuals infected with the parasitic blood fluke S. mansoni. Sixty other patients not suffering from schistosomiasis were studied. The test was also performed in sera from 100 persons before and after administration of variable amounts of stibophen (Fuadin), an antimony compound employed against the parasite. Fifty other cases were used as a contratest group not receiving treatment. Results indicate that the circumoval reaction is a reliable diagnostic tool, but at present it cannot be considered as another criterion to appraise effect of chemotherapy in this disease.