The understanding of the mechanism(s) of multidrug resistance has increased at a rapid rate over the past year. Many new inhibitors, new agents equitoxic to MDR and sensitive cells, techniques for screening, and application to therapies will no doubt be within the grasp of the clinical oncologist over the next few years. However, whether in-vitro MDR is synonymous with clinical resistance is still an unanswered question, and other forms of resistance may be co-existing in patient tumors.