The dosimeter method of administering doubling cumulative doses of methacholine to measure bronchial responsiveness was standardized to control for the effects of a number of potential influencing variables. The aerodynamic mass median diameter of the challenge aerosol produced from a DeVilbiss 646 nebulizer proved to be 1.2 mu, and the mean output per inhalation 8.9 microliters. Each challenge dose comprised 5 inhalations. Cumulative doses ranged from 0.3 methacholine inhalation units (1 unit = 1 inhalation of aerosol from a 1-mg/ml solution of methacholine, i.e., 8.9 micrograms methacholine) to a possible 640 units, the maximum that was considered reasonable to avoid the risk of unacceptable systemic effects. Responsiveness was expressed as the dose provoking a 20% decline (PD20) in FEV1. Modifications in this full protocol were introduced to facilitate epidemiologic investigations. Physician assessments coupled with baseline measurements of ventilatory function allowed starting at higher dosages for persons with low probability of hyperresponsiveness, thereby shortening the time required for testing to an average of 38 min. In a validation study of 20 persons using both the full and modified protocols, no significant differences were detected between measured PD20 values (geometric means Full versus Modified, 14.83 versus 14.88; r = 0.99). The modified protocol was used to measure bronchial responsiveness in 254 workers exposed to toluene diisocyanate. It proved to be safe and acceptable. Sixty-four workers (25.2%) were found to be reactors. The frequency distribution of the PD20 values exhibited a steadily increasing trend, consistent with a unimodal distribution.