The effects of demographic characteristics, exercise, environmental exposures, and other host factors on cellular and biochemical constituents of human bronchoalveolar lavage (BAL) fluids were investigated by studying more than 300 specimens obtained from normal volunteers and assayed in a single center. The BAL data demonstrated associations with race, smoking, exercise, skin-test reactions, and blood constituents, and weak or no associations with age, sex, pulmonary function tests (PFT), or ambient ozone exposure. The effect of exercise was relatively strong and more clearly characterized than in previous studies. Smoking effects were similar to those observed in other studies; our ability to study age and ambient ozone effects was greatly limited because of the homogeneity of the population under study. Blood constituents of the subjects also showed an association with level of exercise. Analysis of intraindividual and interindividual variability in BAL constituents results suggested that matching, although desirable, is not essential for the maintenance of adequate statistical power in BAL studies, so observational studies of the effects from air pollution on BAL fluids in humans could be effectively conducted using cross-sectional designs.