The IgE mediated reactions such as 48 hr homologous passive cutaneous anaphylaxis (PCA) and active anaphylactic bronchoconstriction in rats were inhibited in a dose dependent manner by treatment with 7-acetyl-5-oxo-5H-[1]benzopyranol[2,3-b] pyridine (Y-9000) and disodium cromoglycate (DSCG) given intraperitoneally. The inhibitory activity of Y-9000 was to the same extent as that seen with DSCG. The IgE mediated reactions were also inhibited by oral treatment with Y-9000 but not with DSCG. In addition, the treatment with Y-9000 resulted in inhibition of IgG mediated reaction such as anaphylactic asthma in the passively sensitized guinea pigs and 4 hr heterologous PCA in rats. However, DSCG failed to prevent these reactions. Y-9000 also inhibited the active systemic anaphylaxis of the mouse and non-immunological reactions in rats such as histamine release after an intraperitoneal injection of dextran, anaphylactoid reaction and paw edema induced by the dextran, egg white or carrageenin. This agent had a stimulating effect on the adrenals, and showed glucocorticoid like activity, but bronchodilator and antagonistic activities on chemical mediators were nil. These results suggest that the anti-allergic activities of Y-9000 are elicited by inhibiting the release of allergic mediators in a manner similar to DSCG, and are partially mediated by stressor activity.