1. A specific complement fixation reaction test for Plasmodium knowlesi malaria in rhesus monkeys is reported with details involved in the preparation of the antigen and procedures employed in setting up the test.
2. It was found that specific complement-fixing antibodies appeared early in the course of the experimental disease and persisted during the course of the chronic infection.
3. The first appearance of complement-fixing antibodies was generally followed by a temporary fall in titer. During the first 2 months of infection there was no apparent relationship between the number of circulating parasites and the changes in complement fixation titer.
4. During the stage of chronic infection there was a fall in the titer of complement-fixing antibodies preceding each parasitic relapse, and after the relapse had terminated, there was an elevation of the complement-fixing titer.