Carpopeltis affinis Okamura (CA, Halymeniaceae) has long been used as therapeutics for various allergic diseases in Korea. The precise effects of CA in experimental models, however, have remained unknown. We studied the effects of a methanol extract of CA on atopic allergic reaction. Histamine content was measured by the o-phthalaldehyde spectrofluorometric procedure. Cytokines were measured by a modified enzyme-linked immunosorbent assay. Cytotoxicity was determined by the 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. CA significantly inhibited the histamine release and beta-hexosaminidase release from rat peritoneal mast cells. CA also inhibited interleukin-8 and tumor necrosis factor-alpha secretion from the phorbol 12-myristate 13-acetate and A23187-induced HMC-1 cells (human mast cell line). 48 h exposure to CA (1.0, 10, and 100 microg/ml) had little effect on HMC-1 cell viability. Our results suggest that CA has an inhibitory effect on mast cell-dependent allergic reaction and thus may be useful in the treatment of atopic dermatitis.