Cell kinetic information is an important adjunct to histologically-based tumor classifications. Presently, cell kinetic data can be obtained from slide-based material only with monoclonal antibodies such as Ki-67, which require the use of frozen sections and cannot be applied to archival, paraffin-embedded material. Monoclonal antibodies have recently been generated to PCNA/cyclin, a 36 kd, S-phase-associated nuclear protein. The authors investigated whether monoclonal antibody 19A2 could be used to identify proliferating cells within fixed, embedded tissue sections. Deparaffinized sections of 41 methacarn-fixed human tumors were immunostained with 19A2 using a streptavidin biotin immunoperoxidase system. A semiquantitative scoring system was used to evaluate the fraction of cells that were PCNA/cyclin-positive, and this score was compared with cell kinetic data obtained from parallel flow cytometric S-phase analysis that had been performed on fresh samples of the same tumors. While there was general agreement between the slide-based, antibody-derived and the flow cytometrically-derived cell kinetic information, some discrepancies were observed. Some of the latter represented cases in which the anti-PCNA/cyclin antibody preparations demonstrated significant heterogeneity in the numbers of proliferating cells in different regions of the tumor. In other cases, a significant fraction of the positive cells corresponded to nontumor stromal and/or inflammatory cells. In these cases, the slide-based method provided more information about the tumor cell population than did the flow cytometry data. It is concluded that semiquantitative immunocytochemical analysis with anti-PCNA/cyclin antibodies may represent a simple, reproducible, yet powerful technique for the routine analysis of cell kinetic data in alcohol-fixed, paraffin-embedded tissue by the surgical pathologist.