BL-5255 exhibited potent activity in several models of antigen-induced immediate hypersensitivity reactions in rats and guinea pigs. The compound was effective whether administered by oral or parenteral routes and in passively and actively sensitized animals. It appeared to be readily absorbed when given orally. Localized skin and bronchoconstriction reactions in rats were inhibited by the compound by oral doses at 0.014 and 1 mg/kg, respectively. BL-5255 was protective against both IgE- and IgG-mediated reactions in the rat and guinea pig. Its effectiveness versus the systemic anaphylaxis reaction in the guinea pig appears to be due to BL-5255's ability to inhibit both the IgE and IgG1 components of the reaction.