Ever since Takahara discovered acatasemia in 1946, many studies on the questions concerning causative factors of progressive necrotic ulcers and phsiological significance of catalase have been made public. As there is no report other than that by Miyamoto concerning the relationship between blood of the acatalasemic patient and bacteria and considering this to be an interesting problem, the author conducted a series of experiments with a view to elucidate actions of several bacteria belonging to Streptococcus and Pneumococcus on blood of the acatalasemic patients. Namely, the author carried out aerobic and anaerobic cultures with the medium containing Diprococcus pneumoniae Ⅰ, Ⅱ, Ⅲ, Streptococcus hemolyticus, or Streptococcus viridans, which was loaded with either acatalasemic blood or blood of the normal person, and other series of cultures containing each of the these media loaded still further by inoculation of other medium containing either KCN or hydroxylamine (inhibitory agent of catalase action). By macroscopic and spectroscopic observation on the results of above mentioned expeirments, the following findings were obtained. 1) As regards the rate of bacterial growth under either condition no great difference could be recognized between the medium loaded with normal blood and that with acatalasemic blood. 2) In the cases of aerobic culture, the production of Methb and decolorization in the media containing actalasemic blood tended to be more marked than those in the media with normal blood. 3) In the cases of anaerobic culture, the production of MetHb and decolorization phenomenon decreased in the media containing acatalasemic blood. 4) In the aerobic culture with the medium loaded with inhibitory agent of catalase action, the rate of MetHb production and decolorization phenomenon were about the same in both the medium with normal blood and the medium with patient's blood. While, in anaerobic cultures influences of addition of these agents could hardly be discerned. 5) Since the blood of patient doss not contain catalase, the production of MetHb and decolorization phenomenon in the medium containing acatalasemic blood seem to have occured due to influences of hydrogen peroxide (H(2)O(2)) produced by bacteria.