Summary Tris(p-aminophenyl)carbonium chloride and pamoate, referred to as CI-403 chloride and CI-403 pamoate respectively, were administered to 64 patients with schistosomiasis in an endemic area in Nyasaland. CI-403 chloride and CI-403 pamoate were given in respective total doses of 150 to 539 and 250 to 788 mg per kg during periods of 4 to 14 days. Stool and/or urine specimens were examined in about half of the patients a sufficient number of times during the first few weeks after treatment, for an indication of the effects of the drugs on egg excretion. CI-403 chloride appeared to suppress egg excretion in 12 of 13 patients infected with Schistosoma haematobium and in one of two infected with S. mansoni. CI-403 pamoate appeared to suppress egg excretion in 11 of 13 patients infected with S. haematobium and in each of the 5 patients infected with S. mansoni. There were significantly more gastrointestinal reactions associated with CI-403 chloride than with CI-403 pamoate. The latter was tolerated well in this respect. Peripheral blood studies in 28 patients and urinalyses in 42 provided no indication of toxicity of either salt. The salts, particularly CI-403 pamoate, seem to be worthy of further study as potentially practical antischistosomal drugs.