This U.S. Bureau of Mines study examines the effects of a barium- based fuel additive on the polycyclic aromatic hydrocarbon (pah) content and ames test mutagenic activity of exhaust particulate matter from a diesel engine commonly used in underground mining equipment. The additive, sold for smoke suppression as lubrizol 565 and containing 20 to 25 pct barium, was tested at three concentrations in the fuel: 0.75, 1.5, and 3.0 G/l. A deutz f6l 912w six-cylinder, air-cooled, naturally aspirated, indirect- injection engine was operated on a programmed, light-duty cycle, and particulate matter was collected by dilution tunnel sampling using teflon-coated, glass-fiber filters. At the manufacturer's recommended level of the additive in the fuel, 3.0 G/l, particulate emissions were elevated approximately 30 pct for either intake condition (both with p < 0.01). The barium-based fuel additive also produced statistically significant increases (p < 0.05 Or 0.01) In both exhaust mutagenicity (i.e., 160 pct, standard engine air intake condition with 1.5 G/l) and exhaust pah concentration (i.e., 60 pct, restricted intake with 0.75 G/l, and 79 pct, restricted intake with 1.5 G/l). These results suggest that the barium additive should not be used for smoke suppression under light-duty operation. Additional studies are needed to evaluate the additive's effects in heavy-duty work cycles.