In order to determine the effect of nucleic acids on the hematopoietic functions of bone marrow, the author added RNA and DNA at various concentrations directly to normal rabbit bone marrow being cultured by the coverslip method, and obtained the following results. 1. The direct addition of RNA at optimal concentration to the culture medium increased the growth area markedly and also there could be observed an increase in the cell density. In addition, there were recognized a marked acceleration of carbon particle phagocytotic ability in pseudo-eosinophils as well as a slight increment in the wandering velocity of these cells. 2. In the case of the addition of DNA there were recognized a mild increase in the growth area and an increase in the cell density. Moreovr, the carbon particle phagocatotic ability of pseudo-eosinophlis was greatly accelerated but there could be seen no effect on the wandering velocity.