Repeated dosing of mice with benzene led to a dose-related decrease in red cell production as measured by the incorporation of 59Fe into developing erythrocytes. Phenol, catechol, and hydroquinone were observed in the urine, largely conjugated with glucuronic acid and ethereal sulfate. During repeated dosing, toluene-soluble radioactivity derived from labeled benzene was found to accumulate in blood, liver fat, and, most significantly, bone marrow. Greater accumulation was observed when water-soluble metabolites of benzene were examined in these organs. Covalent binding of benzene metabolites was also observed in liver and marrow during repetitive treatment. Both covalently bound and soluble metabolites accumulated in bone marrow, liver, and kidney over a 24-h period after a single administration of benzene. The highest levels of covalent binding were seen in kidney and liver after 3 d of dosing at 880 mg/kg, two doses per day. Studies in vitro demonstrated the necessity for metabolic activation to produce covalent binding from benzene. These studies demonstrate that increasing benzene toxicity during repetitive treatment of mice is accompanied by increases in the levels of both water-soluble and covalently bound benzene metabolites.