The NYU PM Center subchronic animal inhalation study addressed the issues of composition and sources of ambient fine particulate matter (PM_2.5), as well as the relationship of these PM_2.5 characteristics to the cellular response of human bronchial epithelial cells. In this simultaneous study, we used an in vitro exposure technique to compare the daily variations of the responses of cells to fine concentrated ambient particles (CAPs) collected from a rural area upwind of New York City for the period of 9 a.m. to 3 p.m. on weekdays only, March–September, 2003. Chemical composition data for CAPs were modeled using factor analysis with varimax orthogonal rotation to determine four particle source categories contributing significant amount of mass to CAPs at Sterling Forest (Tuxedo, NY). These source categories are: (1) regional secondary sulfate characterized by high S, Si, and organic carbon (OC); (2) resuspended soil characterized by high concentrations of Ca, Fe, Al, and Si; (3) oil-fired power plants emissions of the eastern United States identified by presence of V, Ni, and Se; and (4) unknown other sources. To estimate the mass contributions of each individual source category, the CAPs mass concentration was regressed against the factor scores. Regional sulfate was the largest contributor to mass (65%), followed by soil (20%), residual oil combustion (2%), and the other sources contributing 13%. Based on an evaluation of the cellular responses to CAPs and a detailed chemical characterization of the ambient PM_2.5, we investigated the sources and individual components of ambient PM_2.5 that are responsible for the induced cellular response. Nuclear Factor kappa B (NF-κ B) was selected as a monitor of cellular stress response that followed after the exposure to CAPs. The results of the NF-κ B assay were found to be most highly correlated with Ni and V among the individual components, and with the residual oil combustion source category.