The acquired immunity of twenty-eight splenectomized and twenty-three intact cattle following primary inoculation with either of two strains (the Y and the R strain) of Babesia bigemina was investigated. Some of these animals were carriers, while others had ceased to be infected. Observations were made on the response of splenectomized animals challenged up to four times with the original infecting (homologous) strain and finally with the heterologous strain.

In the splenectomized animals, a total of sixty-three homologous challenges produced parasitaemias in only twenty-seven instances, and these were generally delayed and of lower intensity than in control animals. Fifteen negative transmission tests performed shortly after thirty-two challenges showed that it was difficult to re-establish the homologous strain in some animals that had achieved self-cure. Substantial homologous immunity had developed within 16 weeks of the primary infection in animals which were no longer carrying B. bigemina.

All twenty-one splenectomized animals subjected to heterologous challenge developed well-marked parasitaemias. Nine of these had been highly immune to homologous challenge 2 weeks earlier. Eight of the twenty-one were carriers at the time of challenge and these showed much lower parasitaemias than the self-cured animals which almost invariably were acutely affected.

Challenges of twenty-three intact steers confirmed that the two strains differed immunologically. Six of twelve challenged homologously failed to show detectable parasitaemia, whereas all eleven challenged heterologously developed parasitaemias.