With the purpose of studying the influences of sulfonamides on the function of pseudoeosinophils of the rabbit bone-marrow, the bone marrow tissue culture (cover-slip method) was conducted. Sulfonamides were added to the tissue culture medium at various concentrations, the effects of these drugs on the wandering velocity and carbon-particle phagocytosis of pseudoeosinophils were observed; 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), slightly suppressed the wandering velocity and carbon-particle phagocytosis of pseudoeosinophils. 2. These sulfonamides showed a greater inhibitory action on the wandering velocity and carbon-particle phagocytosis of pseudoeosinophils at the concentration of more than 20mg/dl.