A Plasmodium knowlesi infection in rhesus monkeys is almost invariably fatal. This infection, however, may be made chronic by the early administration of antimalarial drugs. The animals then will harbor a chronic infection for an indefinite period. The serum taken from monkeys with chronic infection and injected into those suffering from an acute attack was found to have a definite depressing effect upon the course of the experimental disease. In some instances death was prevented and the acute infection changed into a chronic form; in others, the course of the experimental disease was prolonged.

In a similar manner the serum from monkeys harboring a chronic Plasmodium inui infection, when injected into monkeys suffering from an acute attack due to this parasite, was found to be effective in reducing the intensity of the primary infection.

The data presented indicate that protective antibodies are produced in the serum of monkeys during experimental malaria infection.

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