Immunologic and hematologic parameters were used to evaluate the effects in humans of a single 2 hour exposure to either clean air or 0.75 ppm SO2. Venous blood Samples were obtained before, immediately after, and 24, 48, and 72 hours following the end of exposure. Parameters studied included complete blood counts, enumeration of lymphocyte populations using surface membrane receptor markers, evaluation of lymphocyte mitogen stimulated response, and concentration of secretary immunoglobulin A (s-IgA) content of nasal washings. No statistically significant changes were seen in s-IgA, blood erythrocytes or immunologic parameters examined. A possibly significant decrease was found in monocytes 48 hours following SO2 exposure, but this recovered after 72 hours. A stimulatory effect (not statistically significant) was noted at 48 and 72 hours post-SO2 in s-IgA and in lymphocyte numbers bearing receptors for the Fc portion of IgG, while a decrease occurred in active T-lymphocytes with receptors for sheep red blood cells.