A major form of multidrug resistance, which represents a serious obstacle to the success of chemotherapy, is caused by the over-expression of MDR-1 gene encoded P-glycoprotein. The present investigation was aimed to determine whether AZT, a cytostatic agent that interferes with the human immunodeficiency virus replication, is able to induce MDR-1 expression in tumor cells. After a short term exposure of human lymphoblastoid cells to AZT MDR-1 P-glycoprotein was found in the treated cells. This ATP-dependent drug-efflux pump interferred with cytotoxic efficacy of anticancer drugs such as vinblastine. This phenomenon should be carefully considered during anti-viral and anti-tumoral combined chemotherapies in AIDS patients.