AbstractLiver proteins of male C57BL/6T mice treated with 0, 50, or 250 mg/kg Aroclor 1254 were analyzed by high‐resolution two‐dimensional (2‐D) electrophoresis. The resulting patterns were processed using a computerized image analysis system and quantitative data selected for a total of 150 protein spots. On the basis of an analysis of liver proteins form five animals in each treatment group, we found 31 proteins that showed quantitative differences attributable to treatment with chlorinated hydrocarbons at a high level of statistical significance. One of the altered proteins appears to be Mitcon:2, a heat‐shock sensitive mitochondrial matrix polypeptide; another appears likely to be microsomal cytochromeb5. The results indicate that quantitative 2‐D protein mapping may reveal much more detail regarding thein vivoeffects of toxic xenobiotics than has previously been available, and thus allow a more informative approach to the testing of toxic