A five month study was performed to test the effect of a total milk diet on the susceptibility of mice to various doses of a rodent malaria Plasmodium berghei. The developmetn of humoral immunity was followed by quantitation of specific serum immunoglobuline (IgG and IgM) using the indirect fluorescent antiboduy method. High levels of IgG antibodies persisted for 150 days. IgM antibodies were observed only during the first two weeks of infection. It was not possible to detect parasites in peripheral blood smears of these animals throughout the experiment, except during the first two months. Peripheral blood of these mice produced fatal malaria infection when inoculated into recipient mice fed normal diets. The parasite was present in the liver and spleen of milk diet mice during the entire lenght of the study. The results indicate that a milk diet adminstered to mice as the only source of food protected them against fatal malaria infection regardless of the number of parasite inoculated. the continous presence of the parasite in these animals, at subclinical levels, allow the host to develop specific IgG and subsequent resistance to malaria infection. This acquired immunity was still present in the mice at 150 days post inoculation.