By adding 8 different photosensitizing dyes to bone marrow tissue culture (cover slips method). direct effects of these dyes on the growth rate of bone marrow tissue have been studied. 1. Of these photosensitizing dyes NK 243 belonging to the aminovynil series promotes the growth of bone marrow tissue most markedly. 2. Photosensitizing dyes such as NK 19, 2, 9, and 15 help to promote the growth of bone marrow tissue moderately. 3. NK 325 and 91 do not act directly on the bone marrow growth. 4. Trimethincyanine dye, NK 79, inhibits the tissue growth markedly. 5. Correlationship between the chemical structures of dyes and the growth rate of tissue has been studied. As for the type of dye, there are dyes in the aminovynil series and in the cyanine series, that promote the tissue growth. As for the radical nucleus in several dyes belonging to the cyanine series, thiazol and chinolin nucleus, and hepthyl radical in the alkyl group act as to promote the growth of bone marrow. In the halogen series iodine radical promotes the tissue growth more markedly than chlor radical.