Plasma total peroxyl radical trapping antioxidant parameters (TRAP) and their main antioxidant components (vitamin E, uric acid, protein sulfhydryl groups, and unidentified antioxidant proportions) were measured in 12 small cell lung cancer (SCLC) patients receiving combined chemotherapy consisting of vincristine, adriamycin and cyclophosphamide for SCLC. Plasma samples were collected ten times during the first two cycles of chemotherapy. There is previous evidence that many anticancer drugs exert their cytotoxity via free oxygen radicals. We hypothesized that adriamycin-induced, increased oxygen free radical production should decrease plasma TRAP as a consequence of oxidative stress. A statistically significant reduction of plasma TRAP was noted 8 hours after the first adriamycin infusion. A reduction of calculated TRAP (TRAPcalc)--the sum of concentrations of individual antioxidants, corrected by their experimentally-determined stochiometric factors--appeared 3 hours after the first adriamycin infusion and continued for up to 1 week afterwards. This decrease was due to the reduction of ascorbate and urate concentrations. Total TRAP, however, recovered to initial levels after 200 hours, due to an increase in unidentified antioxidants. The second course of adriamycin. These results are in accordance with previous studies showing the formation of oxidants with the use of anthracyclines. Evidence suggests that the as yet unidentified component of TRAP (UNID) increases during the oxidative stress caused by anthracycline based chemotherapy.