The work described in this paper, involved a thorough evaluation of the potential for using a soybean derived methyl ester referred to as Methyl Soyate) as a replacement or blending agent with low sulfur diesel fuel. The project considered the potential for improving emissions from a diesel engine using different blends of methyl soyate. Structural changes such as piston bowl or inejector tip geometry were not to be considered. Only changes in engine calibration were allowed.The chosen test engine was a Navistar T444E HEUI direct injected diesel engine. This engine is a modem, electronically controlled diesel engine that allows considerable flexibility with respect to changing the injection timing and injection pressure calibrations. The following parameters were considered in the evaluation: 1) blends of methyl ester in low sulfur diesel between 0 and 100%, 2) injection timing, 3) Injection pressure, 3) application of an oxidation catalyst, and 4) application of exhaust gas recirculation.During the entire test program, full exhaust gas analysis and particulate emissions testing was conducted. Both gravimetric and Soxhlet extraction (for soluble organic fraction) analyses of the particulate filters were conducted. All testing was conducted on the basis of steady-state test protocols to allow an understanding of the influence of the fuel change as a function of location in the engine map.The results of the program indicate that substantial emissions improvements can be obtained with a relatively small methyl ester fraction within the 13-mode test. In general, it is possible to reduce HC, CO, particulate and NOx emissions (compared to 100% diesel at the standard calibration). Depending upon the particular calibration and blend, fuel consumption similar to the diesel engine is possible on a mass basis. Exhaust gas catalyst, injection timing and pressure variation and EGR results are presented and discussed.