Nine primary human renal-cell tumors (RCT), one lymph-node metastasis, 4 human xenografts of a RCT in nude mice and a rat RCT line were analyzed by flow cytometry (FCM) using propidium iodide for DNA analysis and antibodies to cytokeratin and vimentin in the indirect immunofluorescence technique for labelling of specific tumor-cell populations. By means of 2-dimensional FCM analysis, vimentin- and cytokeratin-positive (tumor) cells were compared and their DNA content and proliferative fraction analyzed separately from those of cytokeratin-negative stromal and inflammatory cells. In primary human RCT, 2 subpopulations of cells were detected and analyzed separately. Small numbers of tumor cells with an abnormal DNA stemline were also detected. In addition, co-expression of intermediate filament proteins of both the cytokeratin and the vimentin types was detected in the aneuploid cell population. Comparison of 2 model systems of RCT with primary human RCT revealed a similar pattern of tumor-cell subfractions within these tumors. The 2-parameter FCM analysis permits the detection of subpopulations in complex cell suspensions and the quantification of these fractions, as well as analysis of their cellular DNA content.