Patients with non-Hodgkin's lymphoma (NHL) recurrent after chemotherapy exhibit clinical characteristics compatible with the phenomenon of multidrug resistance (MDR) and frequently have detectable levels of P-glycoprotein (P-gp). Paclitaxel has been used in recurrent NHL with limited success. To test whether clinical resistance to paclitaxel can be reversed, we treated patients having paclitaxel-resistant NHL with paclitaxel plus quinine and measured the effects of quinine on paclitaxel pharmacokinetics. Eligible patients had recurrent and measurable NHL. Patients initially received paclitaxel, 120 mg/m2(dose determined by a phase I trial of paclitaxel plus quinine), as a 20–24 h infusion every 3 weeks until there was evidence of clinical resistance. Patients then received paclitaxel at the same dose rate plus oral quinine at a fixed dose rate of 400 mg three times each day. Paclitaxel pharmacokinetics were studied in each patient using paired samples from plasma obtained at the end of the 24 h paclitaxel infusion as an estimate of the steady-state drug level. Of 14 patients treated with paclitaxel alone, one patient obtained a partial response (7%). At the time of disease progession, one patient (same patient) obtained a partial response with paclitaxel plus quinine (7%). Steady-state paclitaxel levels were obtained in 12 patients. In 11 of 12 patients the steady-state paclitaxel level was substantially lower with the addition of quinine. The average ratio of end of infusion plasma levels (paclitaxel alone/paclitaxel plus quinine) was 0.6 (range 0.31–0.97) indicating a 40% decrease in paclitaxel levels with the addition of quinine (p= 0.001). We conclude that paclitaxel given by this dose and schedule has modest activity in recurrent NHL. The addition of quinine to paclitaxel also has limited activity, but the combination did reverse paclitaxel resistance in one patient, adding support to the hypothesis that clinical drug resistance can be overcome with chemosensitizers in individual patients. Pharmacokinetic studies indicate that the reversal of drug resistance in this study cannot be attributed to changes in clearance of paclitaxel (which appears to increase with quinine), but more likely to the sensitization of lymphoma cells.