Circulating immune complexes, measured by the C1q binding and Raji cell radioimmunoassays, were detected in 16 of 25 (64%) patients with schistosomiasis alone, in all 13 patients (100%) with schistosomiasis infection associated with prolonged bacteremia by salmonella organisms, and in 15 of 18 (83%) patients with visceral leishmaniasis. The C3 levels in the serum of patients with schistosomiasis, with and without prolonged salmonella bacteremia, were significantly lower in those with renal disease. Further, in patients with schistosomiasis alone, the absence of renal involvement was positively associated with C1q binding within the normal range (P = 0.015) and the presence of IgM rheumatoid factor in serum (P = 0.04). In six of eight patients with visceral leishmaniasis treated with a pentavalent antimonial, there was a fall in Raji cell binding, suggesting indirectly that the parasitic antigen may be involved in the pathogenic immune complexes in serum.