Plasmodium berghei were released from mouse erythrocytes by passage through a French pressure cell. The released organisms were washed and disintegrated; the soluble portion was chromatographed on a Sephadex (G-200) column. The void-volume eluate contained an erythrocyte-free plasmodial fraction which behaved as a vaccine, preventing parasitemia, anemia, and death in mice subsequently challenged with living Plasmodium berghei.