The ability of hexachlorocyclopentadiene to induce specific locus mutations at the TK locus in cultured L5178Y mouse lymphoma cells (Mouse Lymphoma Forward Mutation Assay) was evaluated in the presence and absence of mouse liver S9 metabolic activation (inducer not reported). Based on preliminary toxicity tests, nonactivated cultures were treated with hexachlorocyclopentadiene at concentrations of 0.0000025, 0.0000050, 0.0000100, 0.0000200, and 0.0000400 ul/ml producing a range of 0.3 - 82.0 % growth relative to the solvent (DMSO) control. Additionally, activated cultures were treated with concentrations of 0.00002, 0.00004, 0.00008, 0.00016, 0.00032, 0.00040, 0.00048, 0.00056, 0.00064, and 0.00125 ul/ml producing growth relative to the solvent control ranging from 2.2 - 115.8%. Hexachlorocyclopentadiene did not induce a positive mutagenic response either with or without metabolic activation.