1. Mice were infected with fertile bisexual Schistosoma mansoni and compared with similar animals infected with unisexual worms or sterile bisexual worms.
2. A significant increase in splenic weight occurred in all infected animals.
3. Administration of well-tolerated doses of 6-mercaptopurine abolished the increase in relative splenic weight in animals infected with ordinary S. mansoni.
4. In splenectomized uninfected mice leucocytosis but no other haematological changes developed.
5. In splenectomized mice lower values for packed cell volume were observed 8 weeks after, but not 12 weeks after, infection with S. mansoni.
6. Slight prolongation of the life-span of erythrocytes occurred in splenectomized infected mice.
7. It is concluded that anaemia in schistosomiasis depends to a significant extent on immunity developed to adult schistosomal worms and can develop in the absence of schistosomal ova.
8. The anaemia resulting from such an immune response may be suppressed by administration of 6-mercaptopurine.
9. Such anaemia occurs even in splenectomized mice; thus hypersplenism is not necessary for its development although splenectomy slightly prolongs the erythrocyte life-span.