Treatment of mice with Trichopus zeylanicus leaf resulted in inhibition of antigen-induced degranulation of sensitized peritoneal mast cells. Further, it reduced the ratio of mast cells in the peritoneal exudate cells. The plant drug treatment did not protect mice from E. coli-induced abdominal sepsis. Studies in rats using mesenteric mast cells confirmed the above mast cell-stabilizing property of T. zeylanicus. This activity was found in the butanol fraction of methanol extract of T. zeylanicus leaf. The treatment with this fraction also reduced the number of rat mesenteric mast cells. However, the in vitro treatment of the mast cells with the butanol fraction did not inhibit antigen-induced degranulation of the mast cells.