In hepatocellular carcinoma cell lines, the intensity of staining with the monoclonal antibody C-219 to the multidrug-resistant gene (mdr1) product P-glycoprotein and the intensity of the band at a molecular weight of 170 KDa on Western blot were associated closely with resistance to Adriamycin but not with the resistance to cis-dichlorodiamine platinum (CDDP). In clinical specimens, noncancerous liver tissue was regularly stained with this antibody on the biliary canalicular front of the hepatocyte cell membrane. In liver cancer tissue, however, regular staining as in the noncancerous regions of the liver was observed in only 16% of the patients, irregular staining was seen in only 24%, and no staining was seen at all in 60%. Staining of P-glycoprotein with the C-219 antibody is technically simple and is useful for studying the role of P-glycoprotein in drug-resistant hepatocellular carcinoma.