The problem of the antigenic properties of unicellular chlorophyll-bearing organisms was suggested by the work of Sherwood (1) in which he attempted to immunize rabbits to the chloroplasts of several higher plants. The serum of the treated animals did not bring about release of the chlorophyll from the chloroplasts in the presence of fresh serum or produce any changes detectable microscopically, and agglutinin production was inconstant. Moreover, in extensive unpublished experiments with purified colloidal chlorophyll, he found no evidence of antibody formation. It was thought that a thorough immunological study of some simple chlorophyll-bearing organism might further our knowledge of the antigenic properties of chlorophyll in the chloroplast and, in addition, give valuable information about a group which has been insufficiently studied from this standpoint. Rosenblat-Lichtenstein (2), using pure cultures of several species of Chlorophyceae, showed that specific agglutinating sera of moderately high titer could be produced.