Using a sensitive RNase protection assay, we have measured the levels of P-glycoprotein mRNA in fourteen different Chinese hamster tissues and in three cell lines to determine whether a relationship exists between the level of P-glycoprotein expression and the occurrence of primary and acquired multidrug resistance (MDR) in cancer. P-Glycoprotein mRNA was detected in all tissues, suggesting that the protein is a normal constituent of the cell. High levels of P-glycoprotein mRNA were found in oesophagus, testis and uterus. Intermediate levels in brain, lung and ovary, and a very low level in the adrenal gland, bladder, bone marrow, heart, kidney, liver and spleen. There is no obvious correlation between this expression pattern and the occurrence of primary or acquired MDR.