The effect of chemotherapy and reinfection on circulating immune complexes (CIC), serum IgE, and parasite-specific IgE antibodies was studied in patients concomitantly infected with <i>Schistosoma mansoni</i> and <i>Schistosoma haematobium</i>. Before chemotherapy IgE- and IgG-containing CIC were present in high concentrations in all patients and IgE antibodies to <i>S. mansoni</i> and to <i>S. haematobium</i> in almost 80% of the cases. Serum IgE was excessively high (median 4,900 ku/1), but decreased to half of its initial value 2 months after chemotherapy. When reinfection with <i>S. haematobium</i> occurred onwards from month 4, the serum IgE reincreased and reached almost the pretreatment level at month 16. Effective chemotherapy was also followed by a rapid decrease of IgE-containing CIC and a slower, but significant, decrease of IgG-containing CIC. However, in contrast to IgE and IgE antibodies, the reinfection did not lead to a reincrease of CIC. It was interesting to note that the concentration of IgE as well as that of IgE-containing CIC was significantly correlated to the intensity of the infection. These investigations demonstrated that high levels of IgE- and IgG-containing CIC are present in chronic human schistosomiasis, but that the concentrations of CIC fall to normal values, when the parasites are eliminated by chemotherapy.