The catechol metabolite of the antitumor agent VP-16-213 and the ortho-quinone of VP-16-213--a secondary metabolite formed from the catechol--easily undergo auto-oxidation into a free radical at pH greater than or equal to 7.4. By elevation of the pH from 7.4 to 10, an increase in the production of the free radical was observed, which was accompanied by the formation of products with higher hydrophylicity than the catechol and ortho-quinone, as found by HPLC-analysis. The hyperfine structure of the free radical indicates that it is the semi-quinone radical of VP-16-213. At pH 12.5 a secondary radical is formed from the catechol and the ortho-quinone of VP-16-213 besides the semi-quinone radical. One-electron oxidation of the catechol with horseradish peroxidase/hydrogen peroxide resulted in the formation of the same radical as observed under alkaline conditions and subsequent oxidation to the ortho-quinone. If the ortho-quinone was incubated with NADPH cytochrome P-450 reductase, a free radical was detected by spin-trapping with POBN, but not without spin-trapping. Studies on inactivation of phi X174 DNA by the system ortho-quinone of VP-16-213/NADPH cytochrome P-450 reductase suggest that the semi-quinone radical may play a role in the process of inactivation of DNA.