Agave intermixta Trel. (Maguey) and Cissus sicyoides L. (Bejuco caro) are Caribbean plant species from the Dominican Republic used locally in traditional popular medicine that have shown an antiinflammatory effect in experimental animal models. A phytochemical analysis on these species allowed us the isolation and identification of the steroidal sapogenins hecogenin and diosgenin from Maguey and the hydroxystilbene resveratrol from Bejuco caro. The effects of these plant extracts and their isolated constituents on compound-48/80-induced histamine release from peritoneal mast cells were investigated. Significant inhibition was produced by 0.5 mg mL(-1) of a methanolic extract of Bejuco (41.1%) and by its constituent resveratrol (82.4%) at a dose of 250 microM. However, none of the steroidal sapogenins from A. intermixta showed a significant inhibitory effect on histamine release from mast cells. From these results, it can be deduced that the in-vitro anti-allergic activity towards the release of histamine from mast cells shown by the methanolic extract of C. sicyoides may be mediated by its constituent resveratrol and might contribute to the antiinflammatory activity shown by this species.