Experiments were carried out in mice, cattle, and sheep to investigate the possibility that heterologous immune reactions may occur between the schistosomes prevalent in man and domestic animals in Iran. Immunization with Ornithobilharzia turkestanicum from cattle produced a considerable degree of immunity in mice against challenge with Schistosoma bovis, S. haematobium, and S. mansoni. The results of immunizing cattle with O. turkestanicum, S. bovis, and S. haematobium were even more striking; there was a reduction of 30-40% in the number of adult worms and a proportionally greater reduction in the tissue egg counts. Sheep developed a less marked immunity. Supplementary experiments on homologous immunity showed that mice developed a considerable degree of immunity against S. bovis. The results of the heterologous immunity experiments with S. haematobium and S. bovis are of particular interest as both parasites often occur in the same area and are often transmitted by the same snail host, man and cattle being exposed to the cercariae of both species simultaneously. The reciprocal immunity produced by these infections may be mutually beneficial in limiting the severity of schistosomiasis in man and domestic animals in the endemic areas.