The ability of dimethyl phthalate to induce specific locus mutations at the TK locus in cultured L5178Y mouse lymphoma cells (Mouse Lymphoma Mutagenicity Assay) was evaluated in the presence and absence of Aroclor-induced rat liver S9 metabolic activation. Dimethyl phthalate, diluted with acetone, was incompletely soluble at concentrations above 700 nl/ml without activation and above 500 nl/ml in the presence of metabolic activation. Non- activated cultures treated in duplicate with 200, 300, 400, 500, and 600 nl/ml produced a range of 101.7 to 52.5% relative growth. Since highly toxic treatments were not achieved, a second trialwith 8 non-activated cultures were treated in duplicate with 400, 500, 650, and 700 nl/ml, producing a range of 98.1 to 26.9% relative growth. S9 activated cultures treated in duplicate with 100, 200, 400, 500, and 600 nl/ml produced a range of 80.3 to 14.0% relative growth. Non-activated cultures at 500 nl/ml and 650 nl/ml produced mutant frequencies significantly greater than the solvent control (acetone); however, a dose-dependent response was not observed. All of the activated cultures produced mutant frequencies significantly greater than the solvent control.