Descriptions of changes in hematological indices have contested the premise that the biological effects of suspended particulate matter (PM) are restricted to the lung. Employing approximately 40 hematologic parameters reflecting blood cells, chemistries, mediators, and coagulation factors, we tested the hypothesis that exposure to concentrated ambient air particles (CAPs) can be associated with changes in hematologic indices in normal humans. Twenty healthy young volunteers were exposed to either filtered air (n = 5) or CAPs (n = 15) with a mean PM mass of 120.5 +/- 14.0 microg/m3 and a range from 15.0-357.6 microg/m3. Hematologic indices were measured. Changes in all parameters are expressed as the absolute value either immediately after or 24 h after exposure. Differences between responses of those individuals exposed to filtered air and CAPs were tested using the T-test of independent means. If significant differences between the two groups were suggested by the T-test (p < .10), the relationship was further evaluated employing linear regression techniques. Regression analysis verified significant linear relationships between particle mass the individual was exposed to and (1) decrements in WBC count 24 h later, (2) decreases in lactate dehydrogenase (LDH) concentration 24 h later, and (3) elevations in fibrinogen levels 24 h later. There were no changes in either inflammatory mediators in the blood or indices of coagulation/fibrinolysis other than fibrinogen. We conclude that exposure of healthy volunteers to CAPs can be associated with decreases of both white blood cell (WBC) count and LDH and increased concentrations of fibrinogen in the blood.