Used crankcase oil from automobiles is being used as a heating fuel in space heaters. Since automobile waste oil (AWO) contains heavy metals, such as lead and zinc, emissions from these burners could cause lung injury. We therefore tested the in vivo pulmonary toxicity of respirable particles from an air atomizing waste oil space heater using AWO. Hamsters were exposed by intratracheal instillation to 0.15, 0.75, or 3.75 mg/100 g body weight AWO particles. At 1 day post-exposure the animals were sacrified, their lungs washed with 12 × 3-ml washes of 0.9% NaCl, and their bronchoalveolar lavage fluid (BAL) analysed for cellular and biochemical indicators of damage. Animals were also sacrified at 4, 7, and 14 days after exposure to 3.75 mg/100g body weight. The results were compared with responses produced by exposure to the same quantities of alpha-quartz, a highly toxic mineral dust, and to iron oxide, a non-toxic mineral dust. One day after exposure to AWO there was extensive pulmonary injury. For example, at the 3.75 mg/100g dose level, albumin levels, a measure of oedema, were very elevated (↑60 ×), as were extracellular levels of the cytoplasmic enzyme lactate dehydrogenase (LDH, ↑13 ×). Pulmonary macrophage phagocytosis of gold particles was inhibited (↓.65%). The injury was greater than that produced by alpha-quartz. Assays of BAL at 4, 7 and 14 days post-AWO exposure demonstrated that most indicators rapidly appproached control values. In contrast, alpha-quartz caused a more persistent response as indicated by a prolonged influx of polymorphonuclear neutrophils and a persistent elevation in LDH. These results suggest that an acute exposure to AWO is less likely to cause chronic pulmonary disease than alpha-quartz. To determine the characteristics of the toxic constituents, AWO particulate suspensions in saline were centrifuged to yield samples of leachates (L) and leached particles (LP). Animals were instilled with high dose (3.75 mg/100 g body weight) equivalents of L and LP and the 1 day response was assessed. Both the L and LP preparations were highly toxic, although LP elicited a somewhat stronger response than L. Because the AWO sample contained high levels of certain metals (e.g., Pb 0.8% by weight), we added 10 mM disodium ethylene diamine tetraacetate (EDTA) to chelate divalent cations. Addition of EDTA to L reduced all reponses, although only in situ macrophage phagocytosis returned to control values. In contrast, addition of EDTA to LP reduced only some toxic responses (oedema, bleeding, and inhibition of macrophage phagocytosis). Thus, metals play a greater role in the toxicity of L than of LP. We conclude that combustion of AWO in space heaters produces emissions which could pose a public health problem.