T he antitumor activity of cis-diamminedichloroplatinum II (cisplatin) is believed to be related to its covalent interaction with DNA where a major DNA binding product is an intrastrand N7-bidentate adduct on adjacent deoxyguanosines. A novel immunoassay was used to quantitate this adduct in buffy coat DNA from testicular and ovarian cancer patients undergoing cisplatin therapy. 44 out of 120 samples taken from 45 cisplatin patients had detectable cisplatin-DNA adducts. No adducts were detected in 18 samples of DNA taken from normal controls, patients on other chemotherapy, or patients before treatment. The quantity of measurable adducts increased as a function of cumulative dose of cisplatin. This was observed both during repeated daily infusion of the drug and over long-term, repeated 21-28 d cycles of administration. These results suggested that adduct removal is slow even though the tissue has a relatively rapid turnover. Patients receiving cisplatin for the first time on 56-d cycles, and those given high doses of cisplatin as a "salvage" regimen, did not accumulate adducts as rapidly as patients on first time chemotherapy on 21- or 28-d cycles. Disease response data, evaluated for 33 cisplatin-treated patients, showed a positive correlation between the formation of DNA adducts and response to drug therapy. However, more data will be required to confirm this relationship. These data show that specific immunological probes can readily be applied to quantitate DNA adducts in patients undergoing cancer chemotherapy.