Abstract In the mouse lymphoma forward mutation assay@ in vitro treatments of the mouse lymphoma cell line@ L5178Y@ with API #83-04 Induced no reliable increases in the mutation frequency at the thymidine kinase (TK) locus. The cells were exposed to API #83-04 for four hours In the presence and absence of rat liver S9 metabolic activation and the test material appeared soluble at all concentrations tested. Under nonactivation conditions@ the test material was assayed from 25 nl/ml to 125 nl/ml but the highest concentration was excessively toxic. The remaining five trestments induced a wide range of toxicities without inducing significant increases above the background mutant frequency. In the presence of metabolic activation@ treatments from 25 nl/ml to 150 nl/ml were assayed for mutant induction and a wide range of toxicities was induced. Sporadic increases in the mutant frequency were Induced@ but the Increases were borderline and not dose-dependent. API #83-04 was therefore evaluated as inactive In the mouse lymphoma cell system.