Methyl tertiary-butyl ether (MTBE) is an oxygenated fuel additive that is present in gasoline at levels up to 15% by volume. Since the 1990 Clean Air Act amendments require the use of oxygenated gasoline in 39 areas of the USA, the use of MTBE is projected to continue to dramatically increase. As the use of MTBE increase, the potential for environmental release of MTBE from gasoline stations and automobiles will also increase. Despite its growing use as a fuel additive and its potential for increased exposure to the public, few genotoxicity data on MTBE have been published in the peer-review literature. In the present study, we tested the potential genotoxicity of MTBE in two short-term test systems, an in vitro Salmonella microsuspension assay and an in vivo mouse bone marrow micronucleus test. For the microsuspension assay, MTBE was tested at 7 dose levels of 30 to 7400 micrograms/tube in tester strain TA98, TA100, TA104, and TA1535, with and without the addition of metabolic enzymes (S9) at 4 concentration (0, 300, 600, and 1200 micrograms S9/ml final concentration). A closed system was used to minimize loss of MTBE. The response was not significant. However, a high degree of toxicity was observed at the highest doses in all tester strains. MTBE was also tested for clastogenicity i the mouse bone marrow micronucleus test using both male and female Swiss-Webster mice. Mice were administered single intraperitoneal injections of MTBE in olive oil at 5 doses ranging from 0.25 to 1.75 g/kg. There were no significant increases in micronucleus formation at any dose of MTBE when compared with the negative control animals receiving only olive oil. MTBE was not positive when tested for point mutations and clastogenicity, using respectively, a Salmonella microsuspension assay and the mouse bone marrow micronucleus test.