To evaluate health risks associated with exposure to particulates in the environment, it is necessary to quantify the bioavailability of carcinogens associated with the particulates. Direct analysis of bioavailability in vivo is most readily accomplished by adsorbing a radiolabeled form of the carcinogen to the particulate. A sample of native diesel particulate collected from an Oldsmobile diesel engine that contained 1.03 micrograms benzo[a]pyrene (BaP)/g particulate was supplemented with exogenous [3H]-BaP to produce a particulate containing 2.62 micrograms BaP/g. To insure that elution of BaP from native and [3H]-BaP-supplemented particulate was similar, in vitro analyses were performed. When using phospholipid vesicles composed of dimyristoylphosphatidylcholine (DMPC), 1.52% of total BaP was eluted from native particulate into the vesicles in 18 hrs; from [3H]-BaP supplemented particulate, 1.68% was eluted. Using toluene as eluent, 2.55% was eluted from native particulate, and 8.25% from supplemented particulate, in 6 hrs. Supplemented particulate was then instilled intratracheally into male Sprague-Dawley rats and distribution of radioactivity was analyzed at selected times over 3 days. About 50% of radioactivity remained in lungs at 3 days following instillation, with 30% being excreted into feces and the remainder distributed throughout the organs of the rats. To estimate the amount of radioactivity that entered feces through swallowing of a portion of the instilled dose, [3H]-BaP-supplemented particulate was instilled intratracheally into rats that had a cannula surgically implanted in the bile duct. Rate of elimination of radioactivity into bile was monitored; 10.6% of radioactivity was recovered in 6 hr, an amount slightly lower than the 12.8% excreted in 6 hrs into feces of animals with intact bile ducts. Our studies provide a quantitative description of the distribution of BaP and its metabolites following intratracheal instillation of diesel particulate. Because rates of elution of BaP in vitro are similar for native diesel particulate and particulate with supplemental [3H]-BaP, our results provide a reasonable estimate of the bioavailability in vivo of BaP associated with diesel particulate.