Possible induction of lacZ mutation was examined in the liver and testis of Muta™Mouse following the administration of carcinogenic halogenated compounds, namely 1,2-dichloroethane (DCE), 1,2-dibromoethane (DBE), carbon tetrachloride, or 1,2-dibromo-3-chloropropane (DBCP). Slight increases were observed on the mutant frequency in the testis DNA isolated from the mice 14 days after treatment with DBCP at 40 mg/kg or with DBE at 60 mg/kg but not in the liver. Further investigation was necessary to confirm the mutation induction by these chemicals in the testis including experiments with longer sampling intervals. No increase was detected in the frequency following DCE administration of single doses of up to 150 mg/kg or of consecutive injections of up to 280 mg/ kg. Marginal but biologically insignificant responses were observed in the liver from the carbon tetrachloride exposed mice. The present results suggest that these carcinogenic chemicals are less efficient for induction of gene mutation in the liver of Muta™Mouse.