A soluble malarial antigen which fixes complement with immune serum is found in the serum of monkeys infected with Plasmodium knowlesi.

The amount of antigen in the serum is related to the parasite count during the acute phase of the infection. The antigen is not excreted in the urine.

Intravenous injection into normal monkeys of serum containing the antigen stimulates the production of specific complement-fixing antibodies which react with antigens extracted from parasitized cells, as well as with the antigen present in serum obtained during the acute phase of infection.

Monkeys immunized with serum antigen apparently possess very little or no immunity to infection.

The soluble malarial antigen is labile to acids and alkalies, is not destroyed by a temperature of 56°C., and is precipitated, for the most part, in the albumin fraction of the serum by ammonium sulfate.

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