The effects of exhaust gas recirculation (EGR) on heavy-duty diesel emissions were studied at two EPA steady-state operating conditions, old EPA mode 9* (1800 RPM, 75% Load) and old EPA mode 11 (1800 RPM, 25% Load). Data were collected at the baseline, 10% and 16% EGR rates for both EPA modes. The study was conducted using a 1995 Cummins M11-330E heavy-duty diesel engine and compared to the baseline emissions from the Cummins 1988 and 1991 L10 engines. The baseline gas-, vapor- and particle-phase emissions were measured together with the particle size distributions at all modes of operation. The total particulate matter (TPM) and vapor phase (XOC) samples were analyzed for physical, chemical and biological properties.The results showed that newer engines with electronic engine controls and higher injector pressures produce TPM decreases from the 1988 to 1991 to 1995 engines with the solids decreasing more than the soluble organic fraction (SOF) of TPM. The total number of particles increased while the total volume of particles decreased from the 1988 to 1991 to 1995 engines.Use of EGR significantly reduced NOx emissions. At steady-state modes 9 and 11 at 20% EGR rate, NOx emissions were reduced by 56% and 29%, respectively. The particulate emissions increased as the EGR rate was increased, as was expected since fuel injection and the combustion system were not optimized for EGR. At mode 9, brake specific (BS) TPM emissions have increased by 16% and by 57% at 10.8% and 16.8% EGR respectively. At mode 11, BSTPM emissions decreased by 11% at 10.6% EGR and increased by 3% at 16.2% EGR. The SOL portion of the TPM increased with increased EGR for all modes and EGR rates and the SOF portion generally decreased with increasing EGR. EGR decreases the number and increases the volume of emitted particles.The total polynuclear aromatic hydrocarbons (PAH) emissions (vapor plus particle phase levels for a given compound) for fluoranthene, pyrene, and benz[a]anthracene were increased at Mode 11 with the higher EGR rate compared to the lower rate. At mode 9 there was no change in the total emissions. The particle phase emissions of benzo[b]fluoranthene, benzo[k]fluoranthene, and benzo[a]pyrene were decreased at the higher EGR rate at Mode 11, and below detection limits for either rate at Mode 9. The mutagenic activity of the SOF increased with EGR use, but the activity of the XOC decreased with EGR use.