Infections of normal ICR mice with the NYU-2 strain of Plasmodium berghei (Pb) are uniformly fatal. However, a proportion of mice that have been vaccinated with a formalin-killed antigen prepared from the blood stages of Pb survive an otherwise lethal challenge. Such immunity is not induced by immunization with normal mouse erythrocytes. The level of acquired anti-malarial immunity is related to the size and number of doses of antigen, and intravenous injection is superior to the subcutaneous route of vaccination. The addition of the adjuvants BCG and Corynebacterium parvum to the immunizing regimen improved the level of protection to a variable extent, depending on the batch of plasmodial antigen with which they were used. The adjuvants were most efficacious when used with batches of antigen which were poorly protective when used alone. These adjuvants were found never to protect ICR mice against Pb unless used in combination with specific antigen.