Abstract In the mouse lymphoma forward mutation assay@ in vitro treatments of the mouse lymphoma cell line@ L5178Y@ with API #81-08 induced no significant increases in the mutant frequency at the thymidine kinase (TK) locus. The cells were exposed to API #81-08 for four hours in the presence and absence of rat liver S9 metabolic activation and the test material was soluble at all concentrations tested. Under nonactivation conditions@ API #81-08 was assayed at an applied concentration range of 12.5 nl/ml to 300 nl/ml without inducing significant increases in the mutant frequency. A wide range of toxicities was induced. In the presence of the metabolic activation mix@ API #81-08 was also assayed from 12.5 nl/ml to 300 nl/ml. Low to very high toxicities were induced and@ no significant@ dose-dependent increases in the mutant frequency were observed. API #81-08 was therefore evaluated as inactive in the mouse lymphoma cell system.