Proliferating cell nuclear antigen (PCNA)/cyclin is an intranuclear polypeptide antigen that is found in both normal and transformed proliferating cells. We have recently described two mouse monoclonal antibodies reacting with PCNA. In this report we describe the application of these antibodies to the study of proliferating human cells by indirect immunofluorescence microscopy and by flow cytometry. A fixation/permeation procedure was developed in order to obtain satisfactory binding of monoclonal PCNA-specific antibodies to proliferating cells. This method involved fixation with 1% paraformaldehyde followed by methanol treatment. For the staining of cells in suspension with the IgM type monoclonal antibodies lysolecithin was added to the paraformaldehyde solution to achieve a better permeation by the antibody molecules. This procedure gave a good ratio of specific staining relative to the background staining. It also preserved the shape and normal architecture of the cells as judged by visual microscopic observation and by light scatter measurements using a flow cytometer. Furthermore, this fixation technique permits simultaneous labeling of DNA by propidium iodide and PCNA by monoclonal antibodies. PCNA was detected in various types of normal and transformed proliferating cells by indirect immunofluorescence. Quiescent peripheral blood mononuclear cells were PCNA-negative whereas a fraction of lectin-stimulated lymphocytes became PCNA-positive. Similarly, early passages of fetal skin fibroblasts were PCNA-positive but non-proliferating senescent fibroblasts of later passages were PCNA-negative. The association of PCNA-staining by monoclonal antibodies with cell proliferation was confirmed by flow cytometry. Simultaneous labeling of PCNA and DNA showed that the PCNA signal increased during the G1 phase of the cell cycle, reached its maximum in the S-phase, and declined during the G2/M phase. Using cell sorting we demonstrated that mitotic cells had a very low PCNA signal. Thus, monoclonal PCNA-specific antibodies offer a convenient tool for the detection of human cell proliferation by immunofluorescence microscopy and by flow cytometry.