Natural gas and propane were evaluated with reference to a certified DF-2 control fuel for use in a light-duty IDI Diesel engine. The fuels were compared on the bases of performance, combustion characteristics, gas-phase emissions, and particulate emissions. The biological activity of the soluble organic fraction (SOF) of the particulate emissions was assessed using the Ames Salmonella typhimurium TA98- test. Aldehyde emissions were measured using the DNPH-GC method. The methane fraction of the total hydrocarbon emissions was measured for the natural gas fumigation and, correspondingly, the methane, ethane, propane, and propylene fractions were measured for propane operation.The engine could not be run when fueled with only natural gas or propane. At light engine loads, maximum fumigant energy input rates in excess of 85% of the total energy input rate were obtained for natural gas fumigation, while only 75% of the total could be supplied through propane fumigation. At full load, these percentages decreased to 47% and 29%, respectively.Performance and the gas-phase emissions in general did not change appreciably with the introduction of gaseous fumigants at the full-load conditions. However, as load was decreased, performance deteriorated and the gas-phase emissions (with the exception of NOX) increased considerably. Fumigation was found to decrease particulate emissions by almost 50% at full load, whereas they were increased by over 70% at lighter loads. The mutagenic activity of the SOF was about the same or lower than the baseline, except for propane fumigation near full load, which was as high as 1.25 times that of the baseline.