Alpha-, beta-, and gamma-hexachlorocyclohexane (alpha-, beta-, gamma-HCH) isomers are widespread environmental pollutants; alpha-HCH can cause liver tumors in rats and mice. In the present study we have checked first the tumor-initiating activity of HCH using the appearance of phenotypically altered foci in female rat liver as an end point. Foci were identified by means of the gamma-glutamyltransferase (GGT) reaction and by morphological alterations. No evidence of initiating activity was found. Secondly, we have attempted to determine quantitatively the ability of HCH isomers to promote tumor development. For this purpose growth and phenotypic changes of foci were used as an end point. Rats received a single dose of N-nitrosomorpholine. Then five different doses of each HCH isomer or phenobarbital (PB) (as a positive control) were administered continuously via the diet for 4, 15, and 20 weeks. Both number and size of altered foci were enhanced by doses of 2 to 3 mg/kg or more of the three isomers; in addition, foci phenotypes showed a pronounced shift towards strong expression of GGT and sharp demarcation from the surrounding liver. Based on daily doses the three HCH isomers were approximately equipotent; based on concentrations in liver or adipose tissue, gamma-HCH was severalfold more effective than alpha- and beta-HCH. Thirdly, size and DNA and monooxygenase activities of the liver were determined. All three parameters were enhanced by HCH isomers and PB. However, no strict correlations were found. Rather, at the highest doses tested PB was the most effective inducer of monooxygenases, alpha-HCH was the most potent inducer of liver growth, and all three HCHs were more potent than PB as inducers of focal expansion. Thus, induction of liver growth appears to be associated with foci expansion (tumor promotion); however, neither liver growth nor monooxygenase induction can be used for quantitative predictions of foci expansion by chemical compounds. Finally, no-observed-effect levels were estimated for the parameters studied and are discussed in relation to human exposure.