Inhaled beclomethasone dipropionate aerosol is a topically active corticosteroid that has proved to be of great value in treating asthma. The authors have examined the effect of inhaled beclomethasone dipropionate aerosol on circulating leukocytes and on immunological measurements in normal adult subjects. Subjects inhaled either 400 micrograms or 1600 micrograms as a single dose. White blood cell counts, total neutrophil counts, total eosinophil counts and total lymphocyte counts were determined at 0, 2, 4 and 6 hours following an 8:00 a.m. inhalation. Among the 11 subjects who inhaled 400 micrograms, the total white blood cell count increased significantly at six hours (p less than 0.05). The total neutrophil count was increased significantly at 2, 4 and 6 hours (p less than 0.05). Total eosinophil counts and total lymphocyte counts were diminished but not significantly. Among the five subjects inhaling 1600 micrograms similar findings were observed. Seventeen volunteers inhaled 200 micrograms of beclomethasone dipropionate qid for 24 hours. In addition to the above studies, T and B cell numbers were determined and lymphocyte transformation studies were performed. Although trends similar to those observed with single larger dose inhalations were seen, the changes were not statistically significant. The results of these studies indicate that inhaled beclomethasone dipropionate does have the potential to affect circulating leukocytes.