The aim of this investigation was to find out whether resistant cells of different tumors can be detected immunocytochemically by the streptavidin-biotin-peroxidase-complex method using the monoclonal antibody 265/F4. This antibody was prepared against the membrane P-glycoprotein of Mr 170 kd from colchicine-resistant CHO cells. For this purpose the acquired resistance of tissue culture cells, ascites tumors and the acquired and inherent resistance of human lung carcinoma xenografts were analyzed. Doxorubicin-resistant S180 cells, daunorubicin-resistant L1210 cells and vincristine-resistant human epidermoid lung carcinoma xenografts showed an intense positive reaction with the monoclonal antibody. In contrast, no specific immunoreactivity was observed with parental (sensitive) tumor cells. These data could eventually provide a prognostic tool for the detection of resistant human tumor cells.