Particle mass and size distribution measurements have been made on the exhaust of an Onan prechamber diesel engine. Seven fuels were examined: no. 1 and no. 2 diesel fuel, 40 and 50 cetane number secondary reference fuels, and no. 2 diesel fuel doped with three different concentrations of Lubrizol 565, a barium-based smoke suppressant. The no. 1 and no. 2 diesel fuels and the 50 cetane number reference fuels produced very similar emissions with emission indices in the range 0.3-1.3 mg (gm-fuel)-1 and volume mean diameters between.09 and 0.15 μm. The 40 cetane number reference fuel produced both smaller emission indices, 0.2 to 0.8 mg (gm-fuel)-1, and particle diameters, 0.03 to 0.09 μm. These reductions were apparently related to the longer ignition delay period of the 40 cetane number fuel, which allowed better mixing of the fuel and air prior to combustion. Doping no. 2 diesel fuel with Lubrizol 565 led to decreases in particle size and Bosch smoke number, but in most cases, the emission index increased. The additive apparently suppresses the production of light absorbing carbon particles, but at the same time leads to the formation of dense but only weakly light absorbing barium-containing particles.