Bronchial responsiveness to methacholine was measured in 60 swine confinement building workers selected from 461 subjects who participated in a recent epidemiologic survey on the respiratory impact of exposure to this environment. Subjects were divided into the following four groups: group 1, asymptomatic with normal spirometry (n = 16); group 2, asymptomatic with forced expiratory volume in one second/forced vital capacity (FEV1.0/FVC) < 95% predicted (n = 17); group 3, presence of chronic bronchitis with normal respiratory function (n = 13); and group 4, both symptomatic and FEV1.0/FVC < 95% predicted (n = 14). Each subject answered a questionnaire and had a physical examination, PA and lateral chest films, lung volumes and DLCO measurements, and a methacholine bronchoprovocation test. Ages were similar in each group. Physical examination, chest x-rays, and DLCO were normal in all subjects. Values of total lung capacity (TLC) for subjects in group 4 were higher than those of subjects in group 3, and respiratory volume (RV) values of group 4 subjects were higher than those found for all other groups. The provocation dose of methacholine (PC20) was lower for group 4, and the number of subjects with PC20 < 16 mg/ml was greater in this group than in the other groups. We conclude that only swine confinement building workers with both symptoms of chronic bronchitis and abnormal spirometry have an increased bronchial responsiveness to methacholine; however, it is uncertain whether the bronchial responsiveness demonstrated is directly related to the subjects' employment.