We used a recently developed sandwich enzyme-linked immunosorbent assay to determine the kinetics of Schistosoma mansoni circulating soluble egg antigen (CSEA) after chemotherapy and compared these with previously determined levels of circulating cathodic antigen (CCA). Urine samples were collected from 35 Egyptian patients with S. mansoni infection before, and one, 3 and 6 weeks after treatment. Thirteen patients were treated with 60 mg praziquantel/kg body weight and 22 patients with 40 mg/kg. Following chemotherapy, the kinetics of CSEA in urine appeared to be clearly different from those of the worm-derived antigen CCA, levels of which decreased markedly within one week after chemotherapy; CSEA levels decreased at a much lower rate. Six weeks after successful chemotherapy, CSEA could still be detected in urine of 7 cases while CCA had already disappeared and no viable egg was found by faecal examination. There was no significant difference between the 2 dose regimens during follow-up in the percentage remaining positive or in the CSEA level. These results suggest that the egg antigens detected are primarily derived from viable eggs in the tissues and might be used as a marker for morbidity.