1. Specific precipitin- and collodion-particle-agglutination tests are described with antigens and sera from ducks infected with Plasmodium lophurae.

  2. A method for production of a relatively stable antigen from the plasma of ducks heavily infected with P. lophurae is outlined.

  3. Antigen is present in two fractions of the plasma from ducks heavily infected with P. lophurae, but the fraction obtained from the second acid-precipitation (pH 3.2) is more potent and stable than that obtained from the first (pH 5.6 ±).

  4. In general, precipitin-formation follows the appearance of parasites in the peripheral blood and thereafter may follow, coincide with, or vary inversely as the parasitemia.

  5. In ducks that recover, it appears that precipitins persist in the blood for 1 to 2 weeks and then are no longer measurable.

  6. Recovery from infection was always associated with precipitin-formation, but precipitin-formation was not always followed by recovery.

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