The effect of terfenadine on histamine release from human basophils and LTC4 production and release from human eosinophils was evaluated. Eosinophils and basophils were obtained by discontinuous gradient centrifugation of the peripheral blood of atopic asthma patients who were off medication. Anti-IgE-induced histamine release from human basophils was significantly inhibited by terfenadine. Maximum inhibition was obtained at 1 × 10–5m terfenadine (percentage inhibition = 57.0 ± 20.1; P < 0.05). However, only the highest dose of terfenadine used in this study, i.e. 2 × 10–5m, significantly inhibited calcium ionophore (A23I87)-induced histamine release from human basophils (percentage inhibition = 40.0 ± 14.6; P < 0.05), and LTC4 production from human eosinophils (percentage inhibition = 59.8 ± 9.9; P < 0.05. These findings demonstrate that terfenadine, in addition to its known antihistaminic property, also has an inhibitory effect on chemical mediator release.