Effects of photosensitizing dyes on the erythropoiesis and the hemoglobin synthesis as well as on the bone marrow respiration as estimated by oxygen consumption of bone marrow with Warburg's apparatus were studied. 1. When dyes such as NK 2, 19, 9, and 243 are added to the culture medium, the erythropoiesis of the bone marrow can be accelerated; and in the case of adding NK 15 and 325 the acceleration of erythropoiesis is slight in degree. The addition of NK 79 or 91 rather decreases the number of red cells. 2. These compounds have been found to have no direct influence on the hemoglobin synthesis. 3. These photosensitizing dyes hardly have any effect on the oxygen consumption of bone marrow. However, it seems that NK 243 and 19 slightly accelerate the oxygen comsumption of bone marrow, whereas NK 79 in contrast has a tendency to inhibit.