We have studied bronchial hyperexcitability to methacholine (defined as a fall of at least 20 percent in FEV1 at 8 mg/ml or lower) in 504 white male grainhandlers working in terminal elevators in the Port of Vancouver. There was a significant association between bronchial hyperexcitability and level of FEV1; for a 10 percent decrease in percent of predicted FEV1, bronchial hyperexcitability was twice as common. Among those with bronchial hyperexcitability, the PC20 was significantly related to the FEV1. Grainhandlers with immediate skin reactivity to common allergens were twice as likely to have bronchial hyperexcitability. There was a significant increase in the prevalence of bronchial hyperexcitability with increasing duration of employment. Grainhandlers with chest tightness and breathlessness were over twice as likely to have bronchial hyperexcitability. We did not observe an increased prevalence of bronchial hyperexcitability in smokers, and there was no difference between age groups. We conclude that the determinants of bronchial hyperexcitability in this population include baseline FEV1, immediate skin reactivity to common allergens, some respiratory symptoms, and cumulative exposure to grain dust.