Helenalin and tenulin injected into CF1 male mice bearing Ehrlich ascites tumors inhibit DNA synthesis and DNA polymerase enzymatic activity in the tumor cells. Helenalin inhibited protein synthesis. Both drugs increased the concentration of adenosine 3',5'-monophosphate, and interfered with glycolytic and mitochondrial energy processes. Cholesterol synthesis was also inhibited, resulting in lower serum cholesterol levels in tumor-bearing animals. Data obtained in vitro indicate that the cyclopentenone-bearing sesquiterpene lactone and related compounds do not alkylate puring bases of nucleic acids but rather undergo a Michael-type addition reaction with the sulfhydryl groups of reduced glutathione and l-cysteine. Thus, the inhibition of cellular enzyme activities and metabolism that has been observed with these drugs might be explained by the occurrence of a Michael-type teaction.