P-glycoprotein (Pgp)-associated multidrug resistance (MDR) is related to intrinsic and acquired cross resistance to anthracyclines, vinca alkaloids, and other antineoplastic antibiotics. Expression of MDR1 is widely considered to play an important role in conferring resistance to adjuvant chemotherapy in women with breast tumor cells in women with disseminated disease, although data supporting this view is, at best, conflicting. The expression of MDR1 gene and its gene product, P-glycoprotein, was investigated in primary and advanced breast cancers (both previously untreated and previously treated on specific treatment protocols) to assess the role of P-glycoprotein in determining responsiveness to adjuvant chemotherapy. Expression was assessed by immunohistochemistry, reverse transcription-PCR (RT-PCR), Northern Blot and Western Blot. MDR1 mRNA was detected in 40% of the breast cancers tested by RT-PCR with 40 cycles of PCR amplification. When reducing the PCR amplification cycles to 28, the MDR1 gene expression signal disappeared from breast cancers of the highest expressers; however, known MDR1 positive control normal tissues, such as adrenal, kidney, and liver continued to show an expression product. Western and Northern blots failed to demonstrate the MDR1 gene product, P-glycoprotein, in these breast cancers. In contrast, physiologic levels of P-glycoprotein was clearly detected in normal adrenal, kidney, and liver by these techniques. Immunohistochemistry confirmed that breast carcinoma cells lacked P-glycoprotein expression; however, interstitial mononuclear cells, morphologically consistent with lymphocytes or macrophages did show immunostaining in some of these breast tumors. MDR1 gene expression identified by RT-PCR was not correlated either with response to paclitaxel therapy (29 patients able to be evaluated, p = 0.34, Fisher Exact Test) or overall survival (32 breast cancerpatients with clinical follow-up information, p = 0.336, log rank). In conclusion, P-glycoprotein was not expressed in breast carcinoma cells at significant levels, although it was expressed in stomal lymphocytes or macrophages. These results suggest that P-glycoprotein does not play a significant role in multidrug resistance of breast cancer. (Lab Invest 1999, 79:271-280).