Abstract In the mouse lymphoma forward mutation assay@ in vitro treatments of the mouse lymphoma cell line@ L5178Y@ with API #83-11 induced significant Increases in the mutant frequency at the thymidine kinase (TK) locus only In the presence of metabolic activation. The cells were exposed to API #83-11 for four hours in the presence and absence of rat liver S9 metabolic activation. The test material was visibly Insoluble at and above 500 nl/ml. Under nonactlvation conditions@ the test material was assayed from 100 nl/ml to 1000 nl/ml and a wide range of toxicities was induced without inducing significant@ dose-dependent Increases above the background mutant frequency. In the presence of metabolic activation@ the test material was converted to a more active form or forms. Treatments from 50 nl/ml to 1000 nl/ml induced repeatable increases in the mutant frequency that ranged from 2.3-fold to 5.9-fold above the background mutant frequency. API #83-11 was therefore evaluated as active In the mouse lymphoma cell system only in the presence of metabolic activation.