Many studies on the hemolytic actions of Pneumonococci and streptococci have been carried out up to date, but they are all on the catalasemic blood while those on the acatalasemic blood are scarce. And ever since the discovery of acatalasemic patients in the department of oto-rhino-laryngology of our school, various clinical studies have been conducted. Prompted by the urgency of studying the latter, the author have conducted the experiments to clarify the hemolytic action of each bacterium mentioned above on the acatalasemic blood. As for the bacteria of the present experiment, Pneumococci type I, II and III, St. hemolyticus (Cook's strain), and St. viridans have been selected. As for the catalase carrier, the blood of man, domestic rabbit, goat and guinea pig have been employed, and for the acatalase carrier, the blood of domestic duck or that of goose. Thereupon the following results have been obtained: 1. In the case of the 4% blood agar medium, the presence or the absence of catalase in the culture medium makes almost no difference in the growth of each bacterium. 2. P(II), P(III), St. hemolyticus and St. viridans form the larger hemolytic and the Met Hb rings in the acatalasemic medium than in the catalase medium. As for P(I), it scarcely forms any hemolytic ring in either medium, but its MetHb ring is the largest of all. 3. In the acatalase carrier medium, the large discolored rings are formed, and this can be construed to be due to a pentdyopent reaction. 4. In the 1%-glucose medium, the growth of bacteria is better and the hemolytic and the discolored rings are far larger. It seems that the formation of MetHb is somewhat inhibited in this medium. 5. In the anaerobic medium, the formation of MetHb is far less than that in the aerobic medium.