A rapid method for determination of non-specific bronchial hyperreactivity was developed. Resistance to breathing was determined by a modified expiratory airway interrupter technique and combined with a dosimeter-controlled nebulizer which made continuous determination of response possible during challenge. The patients inhaled histamine chloride 8 mg/ml at every eighth breath until resistance to breathing (Rt) was increased by 60%. The number of inhalations (NI) or the provocative concentration (PC60-Rt) of histamine increasing Rt by 60% were determined in 68 patients. The new method correlated well to a non-cumulative standard protocol and could be terminated either within 10 min or within 20 inhalations. The results of this new challenge procedure enables us to predict the responsiveness to inhaled histamine precisely enough to separate patients into hyperreactive or normal reactive patients. Furthermore, the repeatability of the new method is comparable or superior to that of standard methods. The 95% confidence interval for the difference between replicates was the observed value +/- 2.4 NI. Thus this new method will be suited for studies of drug modifying effect on bronchial hyperreactivity since individual dose titration is easily performed, and the method could be valuable in epidemiological and occupational surveys as well.