Rhesus monkeys with experimental Plasmodium knowlesi infections of varying duration were treated with sodium sulfathiazole to sterilize the infection and after differing lengths of time were reinoculated intraperitoneally with homologous strains of the plasmodium, for the purpose of determining whether there is any acquired immunity to malaria in hosts from whom all parasites have been removed.

Two monkeys, one receiving sulfathiazole on the 2nd day of acute infection and the other on the 4th day, had no immunity at the time of reinoculation, 3 weeks and 10 weeks later, respectively. Both developed infections which followed the course usual in an acute attack in a normal monkey.

In monkeys which survived acute infection with the aid of immune serum or quinine and in which a naturally acquired immunity had developed to the point where the acute infection was converted into a chronic one, there was an undoubted persistence of partial immunity up to about a year after sterilization of the infection with sulfathiazole, as indicated by recovery of reinoculated animals after mild or moderately severe infections differing widely in characteristics from the infection in the normal monkey.

The end point at which immunity disappears seems to be independent of the length of the chronic infection.

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