Epidemiologic studies report health effects associated with exposure to ambient particulate matter (PM), but underlying biologic mechanisms remain unclear. We evaluated pulmonary and systemic effects in twelve healthy human adult and twelve asthmatic volunteers exposed once for 2 hours in a whole-body chamber to approximately 200 microg/m3 concentrated ambient particles (CAPs) in the fine (< 2.5 microm) size range and once to filtered air. Neither healthy nor asthmatic subjects showed significant changes in symptoms, spirometry, or routine hematologic measurements attributable to CAPs exposure compared with filtered air. Both groups showed CAPs-related (1) decreases of columnar cells in induced sputum after exposure, (2) increases in certain blood mediators of inflammation (ie, soluble intercellular adhesion molecule 1 [ICAM-1] and interleukin [IL] 6 [marginally significant in asthmatic subjects only]), and (3) parasympathetic stimulation of heart rate variability (HRV). In the asthmatic group, systolic blood pressure modestly increased during filtered air exposure and decreased during CAPs exposure, whereas the pattern was reversed in the healthy group. In summary, this study measured a large number and wide range of biologic endpoints on a relatively small number of healthy and asthmatic volunteers and found few biologic endpoints that responded to CAPs and filtered air exposures with significantly different values. However, observed changes in some mediators of inflammation in blood and changes in HRV were consistent with PM-related effects reported from epidemiologic studies. They suggest that exposure to concentrated PM 2.5 pm or smaller in aerodynamic diameter (PM2.5) tends to elicit more systemic than pulmonary effects. This investigation is one of the first to apply concentrator-exposure technology in a high-risk group (subjects with asthma). Further studies of responses to CAPs that involve other biologic endpoints, other PM size modes, and subjects with other risk factors are needed.