The effects of systemic administration of corticosteroids on the bronchoalveolar cell population obtained from guinea pigs by lung lavage were studied. The results of this study demonstrate that in contrast to the marked decrease in the percentage of T lymphocytes in the peripheral blood of animals treated with steroids, there was no significant decrease in the percentage of T lymphocytes in the bronchoalveolar cell population. In addition, corticosteroid administration did not significantly affect either the number of alveolar macrophages obtained by lavage or the Fc receptor activity of these cells. When considered in the context of previously reported data, these results emphasize that caution must be used in reaching conclusions about the integrity of the pulmonary immune response from the results of studies using a bronchoalveolar cell population obtained by lung lavage. This observation must be considered when evaluating the significance of studies of the human pulmonary immune response performed with cells obtained by segmental bronchopulmonary lavage.