Rats were treated with N-nitrosomorpholin and subsequently with phenobarbital. After several weeks of phenobarbital treatment, foci of altered cells and tumors developed in the livers of the animals. In liver tumors, the activity of glutathione S-transferase was higher than in the surrounding tissue and remained elevated even after the withdrawal of the promoter. Like the glutathio-one S-transferase from normal liver, tumor glutathione S-transferase activity is composed of several, predominantly basic, isoenzymes with an apparent molecular weight of approximately 45,000.
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