Monoclonal antibody QCRL-1 is highly specific for a defined linear epitope in a relatively poorly conserved region of the human multidrug resistance protein (MRP). We have used QCRL-1 to examine MRP expression in archival and fresh snap-frozen samples of untreated small cell (SC) and non-small cell (NSC) lung cancers (LCs), as well as normal lung. We found that the majority (87%) of all histological subtypes of NSCLC had detectable levels of MRP in most of the tumor mass. In a substantial proportion of adenocarcinomas (55%) and squamous cell carcinomas (28%), immunoreactivity approached that obtained with the highly multidrug resistant cell line H69AR from which the MRP was originally cloned. Both the level and frequency of MRP expression in untreated SCLC was significantly lower than in NSCLC. The MRP was detectable in only 56% of SCLC tumors and, in most cases, was expressed in small focal clusters of cells. Immunofluorescence studies of tumor tissue and normal lung confirmed the plasma membrane location of the MRP. However, in normal bronchial epithelium and seromucous glands, unlike in tumor cells, the MRP was detected only on basolateral membranes. In addition, strong MRP immunoreactivity was detected in reactive type II pneumocytes present in hyperplastic alveoli, but not in normal type I and type II pneumocytes. No potentially confounding correlation independent of its possible role in drug resistance was observed between MRP expression in untreated NSCLC and any clinicopathological parameter examined, including overall survival.