This paper reports attempts to immunize domestic fowls against mosquito-borne infections of Plasmodium gallinaceum by means of (a) vaccination with inactivated homologous sporozoites; (b) injections of sera, both normal sheep serum, and serum from fowls chronically infected with the homologous Plasmodium, (c) combinations of both sporozoite vaccine and serum.

It was possible to reduce the normal malaria death rate (55.4 per cent) in these fowls by each of the above methods but most markedly by the combined prophylactic treatment. Mortality rates were 21.1 per cent in vaccinated fowls, 16.7 per cent in serum-treated fowls, and 7.3 in those having the combined treatment.

Intensity of infection was measured by counting the percentage of red cells infected each day. It was found that in each group of fowls having prophylactic treatment the average of highest percentages of red cells infected was less than in untreated malarious fowls (30.1 per cent). The average figure was 20.5 per cent in vaccinated fowls, 17.9 per cent in those having serum injections, and 15.0 per cent in those having combined treatment.

The prepatent period was not markedly affected by any of the prophylactic procedures. It averaged 9.1 days in the untreated group, 8.9 days in both the vaccinated and serum-treated groups, and 9.7 days in the group having combined treatment.

The results seemed to demonstrate an interaction of both cellular and humoral agencies in defence against malaria, since the greatest immunizing effect was seen in the series having both sporozoite vaccine and serum injections.