L5178Y cells resistant to the model quinone antitumor agent, hydrolyzed benzoquinone mustard, were four-fold more sensitive to mitomycin C compared to parental cells. Mitomycin C also produced increased DNA-DNA crosslinking in these cells compared to parental L5178Y cells, but did not induce DNA double strand breaks in either cell line. The resistant cells have a 24-fold increased level of DT-diaphorase activity, an enzyme that produces two electron reduction of quinone groups. Dicoumarol, an inhibitor of DT-diaphorase, significantly inhibited crosslinking and cytotoxicity by mitomycin C in the quinone resistant cells. These findings suggest that DNA-DNA cross-linking may be a major contributor to mitomycin C cytotoxic activity in L5178Y cells, and that the hydroquinone of mitomycin C may play a major role in the crosslinking activity of this agent.