In twenty adult asthmatics studied in a stable state, histamine bronchial challenges were carried out at one-week intervals with the Wright nebulizer and with the dosimeter-DeVilbiss apparatus. Dose-response curves were analysed for sensitivity, defined as the minimal histamine dose causing a 20% fall in FEV1 (PC20), and for reactivity, defined as the slope of the dose-response curve once the reaction starts to occur. A significant relationship (r=0·80) was found for the sensitivity obtained with the two nebulizers. PC20 was indeed reproducible within a two folds concentration of histamine in sixteen of the twenty patients. A significant difference (P < 0·01) was found in the reactivity to the two different apparatus, reactivity being greater with the Wright nebulizer in twelve out of fifteen patients so tested. Sensitivity and reactivity showed a borderline relationship (r=0·47) with the Wright nebulizer but not with the dosimeter. A significant correlation (P < 0·05) was found to exist between the initial FEV1 (in % of the predicted value) and the observed sensitivity and reactivity as assessed with the Wright nebulizer, but not as assessed with the dosimeter. We conclude that histamine bronchial challenges with the Wright nebulizer and with the dosimeter yield reproducible results if the threshold of a 20% fall in FEV1 is taken as positive. However, the two methods produce different results in terms of reactivity.