Small-cell lung carcinomas (SCLC) are highly responsive to various chemotherapies. However only a minority of patients benefit from long survival. SCLC patients treated at the Institut Gustave Roussy received a combined chemotherapy (CCAV) including cisplatin, cyclophosphamide (Cpa), Adriamycin (doxorubicin; Adm) and vepeside (VP16). We report here the intrinsic sensitivity of a small-cell lung carcinoma, designated SCLC-6, grafted in nude mice. This xenografted tumour was derived from an untreated patient. The CCAV regimen given to the patient donor of the tumour sample resulted in a complete response followed by recurrence and death, 8 months after the initial cure. The expression of P-glycoprotein encoded by the MDR1 gene was detected with the C219 antibody on the membrane of SCLC-6 tumour cells. When given to SCLC-6-tumour-bearing nude mice, CCAV induced a strong inhibition of tumour growth (84% of growth inhibition, 20 days after start of the treatment), but no cure. Intensification of CCAV doses did not improve the response. The efficacy of individual agents of the CCAV, given at maximal tolerated doses was analysed. Only cisplatin (10 mg/kg) and Cpa (3 x 50 mg/kg) inhibited SCLC-6 growth (79% and 100% inhibition respectively), VP16 (3 x 24 mg/kg) was poorly efficient (42%) and Adm (10 mg/kg) not at all. Two-drug combinations such as cisplatin plus VP16 or cisplatin plus Cpa inhibited tumour growth (81% and 70%, respectively). Curiously, the efficacy of Cpa, given in combination with cisplatin was less than that of Cpa alone. Repeated treatments with CCAV administered to mice at each in vivo passage of the tumour induced a loss of chemosensitivity, which was observed until the ninth passage. An improvement of the therapeutic response was obtained by adding a headline reverser of multi-drug resistance, verapamil (25 mg/kg), to CCAV (81% versus 63% inhibition). MDR1-related resistance appeared to play a role in the failure of SCLC-6 chemotherapy; frequent recurrences after treatment with cisplatin and Cpa, two drugs that are not recognized by the P-glycoprotein, indicated that other modes of resistance were simultaneously active.