In a number of cell lines with a multidrug resistant phenotype, there is no overexpression of the putative efflux pump, P-glycoprotein. Some such lines do, however, overexpress the MRP gene which encodes a protein bearing considerable amino acid homology to P-glycoprotein. We have used in situ hybridisation to study expression of the MRP gene in human cell lines, lung tumours (representing all the major histologies) and normal lung tissue. Considerable heterogeneity of expression was seen in parental cell line COR-L23/P whereas relatively uniform high-level expression was seen in the resistant line COR-L23/R. Normal bronchial epithelium was strongly positive, but the major epithelial component of all eight lung tumours studied showed only a negative to weak signal. However, the leading edge of the tumours consistently produced a more intense signal similar to that in normal epithelium. Areas of lymphocytic infiltrate were more strongly positive than the tumour epithelium. These results suggest that expression of the MRP gene may be a significant factor determining response of lung tumours to chemotherapy, but that considerable caution is needed in the interpretation of expression studies carried out on homogenised tissue biopsies.