Two experiments were conducted to determine the effects of low-level exposure to airborne lead particulates on humans. Male volunteers were exposed 23 hours per day for about 18 weeks to either 10.9 mug Pb/m3 or 3.2 mug Pb/m3 of air. The particulate matter consisted primarily of sub-micron sized particles of alpha lead dioxide. During exposure to 10.9 mug Pb/m3, the mean blood-lead level of the men increased to 37 mug/100 ml while the level in controls remained at about 15 mug/100 ml. Exposure to 3.2 mug Pb/m3 elicited an increase in blood-lead to about 27 mug/100 ml. In both cases, the blood-lead levels appeared to plateau after about three months of exposure and subsequent to that, no further increases were observed. About two months after the men left the exposure chamber, their blood-leads returned to near normal levels. Although urinary excretion of lead was always within the limits usually considered normal, there was a definite increase in urinary lead in the men exposed to 10.9 mug Pb/m3 and a somewhat smaller increase in the men exposed to 3.2 mug Pb/m3. The activity of delta-aminolevulinic acid dehydrase was reduced by about 50% in the erythrocytes of men exposed to 10.9 mug Pb/m3, but returned to normal after exposure was terminated. Practically no changes in the activity of the enzyme were noted in the men exposed to 3.2 mug Pb/m3. Exposure to either level of particulate lead did not cause measurable changes in the excretion of heme precursors nor were there any alterations of serum chemistry or hematology profiles in any of the men.