Protection tests have been utilized to determine the effect of superinfection upon the potency of immune serum of monkeys with chronic Plasmodium knowlesi infections. The results of these tests showed that:

1. In 2 groups of monkeys with comparable P. knowlesi infections the immune serum of 8 monkeys which had been superinfected on 7 separate occasions over a period of 2 months was much more potent than the immune serum of a group of 7 monkeys which were allowed to continue their chronic course of infection without superinfection.

2. After a series of 9 more intense superinfections the serum from the same 2 groups of monkeys contained no demonstrable protective antibodies.

3. The serum from 8 of the 10 monkeys in the original 2 groups showed a relatively high concentration of protective antibodies following a month's rest and a single superinfection.

4. The results of the experiments indicate that it is possible to increase the potency of immune serum by superinfection, but it is also possible to obtain a decrease in the protective property of the serum by too severe superinfections.

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