The effects of 11-oxo-11H-pyrido[2,1-b]quinazoline-2-carboxylic acid (Sm 857), a new antiallergic drug, on immunological reactions in mice and in humans were investigated. The mouse hemolysin formation against sheep erythrocytes was not suppressed by Sm 857 (10-100 mg/kg/d i.p., p.o.) for 3 days either before or after immunization. The formation of anti-dinitrophenyl (DNP) IgE antibodies in mice immunized with dinitrophenyl-conjugated ovalbumin (DNP-OA) and alum, was not suppressed by Sm 857 (10-100 mg/kg i.p., p.o.) for 3 days either before or after immunization, but the formation of anti-DNP IgG antibodies on 7 days after immunization was slightly suppressed in the group of mice treated i.p. with 100 mg/kg/d of Sm 857 for 3 days after immunization. On the other hand, both the proliferation of human peripheral blood lymphocytes (PBL) induced by concanavalin A, phytohemagglutinin-P, pokeweed mitogen (PWM), protein A (Pro A) and Cowan I, and the formation of IgM and IgG plaque forming cells by PBL stimulated with PWM or Pro A, were suppressed significantly only in the presence of Sm 857 (10(-4) g/ml). In addition, Sm 857 hardly affected the nitroblue tetrazolium reduction test, phagocytosis and chemotaxis in human neutrophils.