A monoclonal antibody, MRK 16, specific to a human myelogenous leukemia cell line, K-562, and resistant to Adriamycin, was used to determine the localization of the antigen molecules (P-glycoprotein) recognized by the monoclonal antibody. P-glycoprotein was found to be expressed very strongly in the adrenal cortex and medulla of adults and strongly in the renal tubules of the kidney and the placenta. Interestingly, P-glycoprotein was not distributed in fetal and neonatal adrenals, and thus may be closely related to adrenal maturation. A high level of P-glycoprotein expression was also seen in one case each of untreated lung cancer (one of ten) and breast cancer (one of nine). Immunoelectron microscopically, the P-glycoprotein was distributed evenly on the membranes of K-562/ADM and 2780 cells. These results imply that the presence of the glycoprotein may be useful as a marker for in vitro studies of multidrug resistance in various malignancies and as an indicator of therapeutic efficacy of ex vivo eradication of multidrug-resistant cancer cells, although other mechanisms of drug resistance may exist, and there is a possibility that this MRK 16 monoclonal antibody may not recognize all P-glycoprotein.