With the purpose of studying the influences of sulfonamides on the erythropoietic function of bone marrow, the cell suspension culture of the rabbit bone marrow was conducted. Sulfonamides were added previously to the fluid medium of the bone-marrow tissue culture at various concentrations, and the effects of these drugs on the increasing rate of hemoglobin, erythrocytes and reticulocytes were studied; and the following results were obtained. 1. Sulfonamides such as sulfanilamide, sulfapyridine, sulfathiazole, sulfisoxazole, sulfisomidine and carbutamide, at the concentration equivalent to the therapeutic blood level (1-20mg/dl), exerted a slight inhibitory action on the increasing rate of erythrocytes and reticulocytes but almost no effect on the increase of hemoglobin. 2. These sulfonamides, at the higher concentration than 20mg/dl in the medium, slightly suppressed the increase of hemoglobin.