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In order to evaluate the genotoxic potential of DI diesel engine exhaust, Ames tests based on TA98 and TA100 bacteria strains have been conducted. The presence of nitro- and dinitropyrene was proved by comparison of TA98 to TA98NR and TA98 1,8DNP6, respectively. One light and three heavy duty diesel engines, representing past, present and future production, have been tested under steady-state and transient operating conditions. A correlation between engine load and mutagenic activity has been shown from the particulate extract. Water port injection reveals a dramatic decrease of mutagenicity at high load conditions. In the particulate extract from future very low NOx engines, no nitro- and dinitropyrenes are detected. Engines representing past and present technology however, contain these highly mutagenic compounds. High quality diesel fuel yields another substantial decrease of mutagenicity.