In this study the effect of blending biodiesel (methyl soyester) with conventional diesel on emissions has been investigated. A 1991 MC Series 60 engine was employed and emissions of NOx, CO, THC, and PM were determined using the heavy-duty transient test. The fuels tested were a reference diesel, 20%, 35%, and 65% biodiesel blends in the reference diesel, as Well as 100% biodiesel. These tests show that as the percent biodiesel increased, the NOx, emission increased, while THC, CO and PM decreased. For 35% biodiesel, the composite NOx emission increased by nearly 1% while the composite particulate emission decreased by 26% relative to the reference diesel. The NOx increase of 1% was found to be statistically significant at the 99% level. For 100% biodiesel, the composite NOx increased by 11% while PM was decreased by 66%. CO was reduced by 47% and total hydrocarbon by 44%. For the 100% methyl soyester, the composite NOx emission exceeded the 1991-1994 NOx emission standard of 5 g/bhp-h. The composite PM emission for the 0.25 g/bhp-h engine met the 1994 standard of 0.1 g/bhp-h. Engine efficiency was found to be the same for biodiesel and biodiesel blends as for the reference fuel. Fuel consumption for biodiesel blends can therefore be calculated from diesel fuel economy data. A preliminary emissions model describing the effect of diesel properties and oxygen content (from blending biodiesel) on emissions is presented. This model suggests that relatively easily obtainable reductions in aromatic content or increases in cetane number could lead to NOx, neutral diesel/biodiesel blends as compared to certification diesel fuel.