Tetrachloroethylene (PERC) is believed to exert its adverse effects upon humans via metabolism by the liver. Concern that PERC is likely to be a human carcinogen is based upon the evidence of the National Cancer Institute bioassay, in which PERC induced a statistically significant increase in the incidence of hepatocellular carcinomas in both sexes of B6C3F1 mice. Tetrachloroethylene has not been clearly demonstrated to cause point mutations in bacteria, but it may be genetically active in yeast. The potential of PERC to produce adverse teratogenic or reproductive effects is, at present, undetermined. Of special concern is the observation that PERC is preferentially concentrated in maternal milk. Thus, nursing infants may represent a special risk group. Both acute and chronic exposure levels have the potential to cause liver damage in humans. In animals, the lowest observable adverse effect level is 100 ppm. However, this may not be sufficiently protective of human health with regard to liver and kidney toxicity.