Novelty: New drug resistance agents are provided which sensitize tumour cells to anticancer drugs and/or inhibit the tumour cells' resistance to antitumour drugs. The agents disclosed can be easily prepared synthetically. Biology: The compounds consist of a binary vinca alkaloid and have a different absolute configuration to vinblastine or vincristine at C-14' and/or C-16' The S absolute configuration in the C-16' position is essential for antitumour activity. The modulators investigated, specifically C-20'deoxy-C-20'diethyl vincovaline, are not cytotoxic at up to 25 μM for sensitive cells and 100 μM for resistant cells. © 1992 Informa UK, Ltd.