The report provides a careful analysis summarizing and critically reviewing the less than satisfactory state of information concerning the effects of diesel engine emissions on humans. Materials moderately active as mutagens in various assays and as carcinogens when painted on the skins of susceptible animals have indeed been partially purified from diesel exhausts. However, no evidence of carcinogenesis has been noted in animals breathing diesel exhaust fumes or in epidemiological studies of relatively heavily exposed human populations. Unfortunately, almost all of the studies are reported to have been defective in some manner and, hence, do not permit definitive conclusions at this time. Nor do the limited observations concerning the effects of diesel exhaust emissions on pulmonary physiology, susceptibility to infection, etc., permit definitive conclusions. However, several important, relevant studies are currently in progress. The report offers a well constructed outline of the research required to generate a degree of understanding more nearly adequate to appraise the possible impact on the public health of major transition from gasoline engines to diesel engines in the American fleet of light-duty vehicles.