Recent epidemiologic studies have consistently reported increased daily mortality associated with exposures to particulate air pollution. Currently, particulate mass is measured as particles smaller than 10 microns (PM10). Fine (PM2.5) and coarse (PM10-PM2.5) mass and sulfate particle concentrations were measured in six eastern U.S. cities for eight years, and aerosol acidity concentrations were measured for approximately one year. Daily mortality for these metropolitan areas was combined with particulate air pollution and weather measurements. City-specific associations with each measure of particle pollution were estimated by Poisson regression, adjusting for time trends and weather by nonparametric methods. Combined effect estimates were calculated as the inverse variance weighted mean of the city-specific estimates. PM10, PM2.5, and SO4= were each significantly associated with increased daily mortality, while no associations were found with coarse mass nor with aerosol acidity (H+) concentrations. The strongest association was found with PM2.5. A 10 micrograms/m3 increase in two-day mean PM2.5 was associated with a 1.5% (95% CI 1.1% to 1.9%) increase in total daily mortality. Somewhat larger increases were found for deaths caused by chronic obstructive pulmonary disease (+3.3%) and by ischemic heart disease (+2.1%). These data suggest that increased daily mortality is specifically associated with particle mass constituents found in the aerodynamic diameter size range under 2.5 microns, that is, with combustion-related particles.