Between April and October 1991, eleven farm workers from a Swiss mountain village (altitude 2800 feet) underwent a monthly check on lung function and bronchial responsiveness to methacholine at the end of a normal working day in the open air. The results were analyzed according to the ozone levels measured at a central station in the village and by passive ozone samplers worn by the participants the day of examination. The ozone levels measured at the central station varied between 31 and 109 micrograms/m3 (mean of the 8 hours before the examination) whereas the levels assessed by the passive samplers varied between 15 and 80 micrograms/m3. The lung function values and bronchial responsiveness were stable throughout the year, even in one asthmatic subject. There was no statistical correlation with the ozone levels measured at the central station or with the passive samplers, in spite of the fact that the subjects did strenuous outdoor work. The difference between the ozone values measured by the passive samplers and by the central station suggests that the individual exposure of the subjects was below the theoretical immission levels.