Studies were carried out on the in vitro covalent binding of the carcinogen trichloroethylene (TCE) to liver microsomal preparations and to exogenous DNA. The binding of TCE to liver microsomal proteins of male C57BL/6 X C3H/He F1 (hereafter called B6C3F1) hybrid mice, a species and strain susceptible to TCE-induced liver tumorigenesis, was 46% higher than that of [14C]TCE to microsomal proteins from male Osborne-Mendel rats, a species and strain resistant to TCE-induced hepatocellular carcinoma. The in vitro binding of [14C]TCE to liver microsomal proteins was 37% higher for male B6C3F1 mice; female B6C3F1 mice that have been reported to show a lower incidence of TCE-induced hepatocellular carcinoma than do males. Microsomal proteins from the lung, stomach, and kidney of B6C3F1 hybrid mice also metabolized TCE, as indicated by the covalent binding of [14C]TCE to microsomal proteins from these organs. For rats the binding of TCE to liver microsomal proteins of Sprague-Dawley animals was higher than that of Osborne-Mendel and Fischer 344 rats. Incubation of [14C]TCE with salmon sperm DNA in the presence of microsomal preparations from B6C3F1 hybrid mice resulted in covalent binding of [14C]TCE to DNA. This binding was much higher in the presence of microsomal proteins from male rather than female mice. The binding to DNA and protein was enhanced by in vivo phenobarbital administration. The effects of 1,2-epoxy-3,3,3-trichloropropane on the covalent binding of [14C]TCE to protein and DNA were also examined.