OBJECTIVE The mouse lymphoma forward mutation assay evaluates a test article's mutagenic potential in a specific locus mutation assay using mammalian cells. The objective of the assay is to determine the ability of a test article to induce forward mutations at the thymidine kinase (TK) locus as assayed by colony growth of L517BY TK-*-/- mouse lymphoma cells in the presence of 5-trifluorothymidine (TFT).