American Petroleum Institute's test article 2,2,4- Trimethlpentane (MBA no. T5316) was tested in the L5178y TX+/- Mouse Lymphoma Mutagenesis Assay in the presence and absence of Aroclor induced rat liver S9. In the first assay, the nonactivated cultures were not in the 10% to 50% Total Growth range. Several critical S9 activated cultures were contaminated and the trial was repeated. In the second assay, the nonactivated solvent control cultures produced unacceptably high background mutant frequencies. The S9 activated cultures in this assay were too toxic to clone. In the third assay, the nonactivated cultures that were cloned had been treated with a range of test article concentrations from 3.0 microl/ml to 0.5 microl/ml. These concentrations produced a range in Total Growth of 6% to 61%. The S9 activated cultures that were cloned had been treated with a range of test article concentrations from 2.5 microl/ml to 0.5 microl/ml. These concentrations produced from 36% to 91% Total Growth. Two of the nonactivated cultures that were cloned exhibited mutant frequencies which were at least twice the mean mutant frequency of the solvent controls. The viable counts of these cultures were low due to poor cloning efficiencies thus producing high mutant frequencies.