Pyocyanine, the blue pigment of B. pyocyaneus, can increase the respiration of living cells to a great degree (maximum observed increase 24-fold). The reversibility of its oxidation and reduction is responsible for this. The effect is non-species-specific and has been observed in varying degrees with B. pyocyaneus, Staphylococcus aureus, Pneumococcus Type III, and the red blood corpuscles of rabbits. The effect of pyocyanine is dependent on the presence of another respiratory ferment sensitive to potassium cyanide and carbon monoxide. The increase of respiration induced by pyocyanine is paralleled by an increase in the respiratory quotient. The pyocyanine catalysis is not indiscriminately effective in all oxidations, but only in the oxidation of certain substances closely associated with the bacterial body.