IN view of the limited success of malaria eradication schemes based on concepts of vector control and chemotherapy, serious efforts are being made to investigate the possibility of developing an antimalarial vaccine. So far two classes of vaccine have been under investigation: (1) an anti-sporozoite vaccine designed to prevent malarial infection by blocking the infectivity of sporozoites introduced by mosquito bite^1,2, and (2) vaccines directed against the asexual stages in the blood^3–5. Although the potential value of such vaccines is unquestionable, their realisation faces technical problems⁶. The latest proposition is to induce immunity against the parasite stages which infect the mosquito and by so doing prevent transmission of the disease by the vector. Gwadz has shown that the infectivity of malarious chickens to mosquitoes can be reduced greatly by prior vaccination with formalin-treated or X-irradiated blood infected with the malaria parasite P. gallinaceum⁷. The number of oocysts (the stage of the parasite developing on the wall of the mosquito gut) in mosquitoes fed on vaccinated chickens was reduced by 95–98% below that recorded in mosquitoes fed unvaccinated birds. To achieve this reduction, chickens were given three weekly intravenous inoculations of a total of 4.5 ml of formalin-treated or irradiated blood. Using the same schedule, but with partially purified gametes of the malaria parasite, we have now reduced the infectivity of malarious chickens to mosquitoes at least 99.9% below control levels. To achieve this reduction we had to use a minimum of 0.1 µl of packed cellular material.