The experiments have been chonducted in order to investigate the effects of hydroxylamin and KCN upon Pneumococci, St. hemolytics, and St. viridans in the catalase carrier and the acatalasemic blood; and the following results have been otained: 1. Hydroxylamine (HXA) and KCN at the concentration of 10(-5) Mol. inhibit the catalase reaction of blood cells without interfering the blood respiration. Furthermore, hardly any influence is exerted on the respiration of bacteria at this concentration. 2. When HXA at the above-mentioned concentration is added to K blood, the Met-Hb rings of P(III), H, and Vi become markedly larger than those of the control; and they approach to those of the AK medium. Moreover, the hemolytic rings of each colony become discolorized. 3. In the case when KCN has been added, they give results rather similar to those of HXA, on the other hand, the size of each ring is smaller. 4. HXA and KCN little affect AK blood, and almost no qualitative changes can be recognized. 5. Even in the case when HXA and KCN are added, the Met-Hb formation in the anaerobic medium as compared with that in the aerobic case can hardly be recognizable or extremely little if any. 6. The results obtained on the observation of the Met-Hb formation in the liquid medium, with the aid of automatic spectrophotometer, coincide rather closely to those revealed in the solid medium.