Resistance to multiple chemotherapeutic agents is a common clinical problem in the treatment of cancer. This resistance may occur before primary therapy or be acquired during treatment. We have generated a monoclonal antibody (MAb) (JSB‐I), specific for a conserved epitope on the plasma membrane 170‐ to 180‐kDa glycoprotein, the expression of which is strongly correlated with the degree of multi‐drug resistance (MDR). JSB‐1 strongly binds to both Chinese‐hamster‐derived MDR cell lines and human MDR cell lines, including cell lines derived from lung and ovary. A drug‐sensitive revertant line, and the corresponding drug‐sensitive parent lines, showed only weak reactivity or none at all. JSS‐l reacts strongly to air‐dried or acetone‐fixed cells and therefore has potential value for diagnostic detection of MDR cells in human tumor samples.