The effects of butyl 3'-(1H-tetrazol-5-yl)oxanilate (WP-833), a new antiallergic drug, on immunological reactions in mice and humans were studied in view of possible side effects. Hemagglutinin formation in mice immunized with sheep red blood cells was not suppressed by WP-833 administered i.p. or p.o. for 3 days either before or after immunization. In addition, the same treatments with WP-833 showed no suppression of hapten-specific immunoglobulin E (IgE) or immunoglobulin G (IgG) antibody formation in mice immunized with dinitrophenyl-conjugated ovalbumin mixed with alum. The proliferation of human peripheral blood lymphocytes induced by concanavalin A, phytohemagglutinin-P, pokeweed mitogen, protein A or Cowan I was not suppressed by WP-833. Nitroblue tetrazolium reduction test, phagocytosis and chemotaxis in human neutrophils were not inhibited by WP-833.