Comparison of various antigenic preparations of Toxoplasma and of the methods for red cell sensitization with these antigens has shown that sensitization with Toxoplasma summary fraction, containing the cytoplasmic and envelope antigens, and rivanol, is the optimal method for preparing an erythrocytic diagnostic agent. A test with such a diagnosticum has proved to be much more effective than the complement fixation test. Though the results of the two tests correlate, this correlation is curvilinear, and therefore it is desirable to use both the tests for the diagnostic investigations.