The effect of some barbituric acid derivatives on primary antibody response to heterologous erythrocytes and phagocytic activity of the reticuloendothelial system in mice has been studied. 5 out of 6 compounds tested here were able to reduce the carbon clearance from the blood stream. However, only 2 out of 15 substances, 5-ethyl-5-phenyl-barbituric acid and l,3-bis-piperidinomethyl-5-ethyl- 5-phenyl-barbituric acid, produced a marked suppression of hemagglutinin titers when given intraperitoneally prior to and after the antigen stimulus.