Previous studies have shown that when mice and monkeys are infected with bovine schistosomes they develop a considerable degree of heterologous immunity against subsequent challenge with Schistosoma mansoni. The present report describes a study on the reverse effect in which calves were first exposed to cercariae of S. mansoni and then challenged with cercariae of S. mattheei. The calves developed patent infections with S. mansoni, excreting viable eggs in the faeces, and it is suggested that cattle may be a source of infection to man under natural conditions. The immunizing effect of exposure to S. mansoni was demonstrated by a reduction in the S. mattheei egg load and adult worm burden in the immunized, as compared with the control, animals. The results suggest that previous exposure of cattle to S. mansoni may reduce the severity of bovine schistosomiasis in endemic areas.