STUDIES with avian malaria have shown that killed sporozoites as well as sporozoites inactivated with ultraviolet light can produce a partial immunity after injection into birds1,2. On the other hand, attempts to use the erythrocytic stages of the parasite as the source of antigen have met with only limited success with avian3, rodent4 and monkey malaria5,6. Previous attempts to use killed sporozoites of the rodent malarial parasite, Plasmodium berghei, to immunize rodents have been unsuccessful. We therefore sought to determine whether protective immunity to this parasite could be achieved by partial inactivation of the injected sporozoites as opposed to injection of dead parasites. X-irradiation was chosen as the inactivating agent, because of the partial immunity obtained by vaccination with irradiated blood forms of malaria parasites7–9. This communication reports preliminary results on the production of protective immunity in mice by vaccination with X-irradiated sporozoites of P. berghei.