This study evaluated the effect of a single exposure to sulfuric acid aerosol on six human biochemical blood parameters. A total of 18 human subjects were exposed to ambient air for 4 hr on the first day of exposure and to 100 micrograms/cu. m. (0.033 micro M) sulfuric acid aerosol (0.5 micrometers mean mass diameter) for 4 hr on the second day. A total of 17 human subjects were exposed for 4 hr to ambient air on both exposure days. On each exposure day, at 30 min and 90 min after exposure was initiated, all subjects engaged in a 15-min exercise period (walking 4 mph on atreadmill inclined at 10 degrees). Six biochemical blood parameters were measured before and after exposure: glutathione, lysozyme, glutatione reductase, serum glutamic exaloacetic acid transaminase, serum vitamin E, and 2,3-diphosphoglyceric acid. The results indicate no significant effect of one 4-hr exposure of humans to 100 micrograms/cu. m. sulfuric acid aerosol. One significant post-exposure effect did occur--an increase in glutathione reductase--for both the control and acid-exposured group. It is concluded that at the levels studied, sulfuric acid aerosol is neutralized in the upper airways; consequently, no cellular damage is measurable.