Information about health effects from controlled exposure to particulate matter (PM) air pollution is relatively limited but potentially critical in urban locations such as Los Angeles, where abundant mobile sources generate combustion-related particles. Nonsmoking healthy (n = 12) and asthmatic (n = 12) volunteers, age 18-45 yr, were exposed to concentrated ambient particulates (CAP) in the fine (PM(2.5)) size range at an average concentration of 174 micro g/m(3) (range 99-224), and to filtered air (FA). Exposures used a two-stage Harvard virtual-impactor concentrator and whole-body chamber and lasted 2 h with alternating rest-exercise periods. Neither group showed significant (p <.05) changes in spirometry or routine hematologic measurements attributable to CAP exposure, relative to FA. Both groups showed CAP-related decreases of columnar cells in postexposure induced sputum, slight changes in certain mediators of blood coagulability and systemic inflammation, and modest increases in parasympathetic stimulation of heart rate variability. Systolic blood pressure decreased in asthmatics and increased in healthy subjects during CAP exposure relative to FA. Cardiovascular (but not respiratory) symptoms increased slightly with CAP in both groups. In summary, the urban fine PM exposures elicited different biologic endpoints with statistically significant differences between CAP and FA. The observed changes in blood inflammation and heart-rate variability were consistent with systemic (rather than respiratory) effects reported from other laboratory and epidemiologic studies. Further studies involving other biologic endpoints, PM size modes, and risk factors will be needed to clarify these results.