The monoclonal antibody Ki-67 reacts with a human nuclear cell proliferation-associated antigen that is expressed in all active parts of the cell cycle. Recently we have raised monoclonal antibodies, MIB 1-3, against recombinant parts of the Ki-67 antigen. These antibodies are true Ki-67 equivalents, as demonstrated by immunostaining of fresh specimens, biochemistry, and molecular biological techniques. Formalin-fixed, paraffin-embedded sections routinely processed for immunohistochemistry failed to stain for Ki-67 and MIB 2. Antibodies MIB 1 and MIB 3 labelled mitotic figures, while non-mitotic proliferating cells were negative under these conditions. However, when dewaxed microwave oven-processed paraffin sections of formalin-fixed tissues were used, MIB 1 and MIB 3 gave strong nuclear staining of those cells presumed to proliferate under a variety of normal and neoplastic conditions. Moreover, routine decalcification or depigmentation techniques did not alter the immunoreactivity of MIB 1 and MIB 3 with microwave-processed paraffin sections. This method is highly reproducible, easy to perform at low cost, and no additional technical skill is needed because after microwave treatment just routine immunohistochemical methods are used. Since we have successfully applied this new method to sections obtained from paraffin blocks stored for a long time (in one case more than 60 years), the assessment of cell kinetics through the detection of Ki-67 antigen is now possible on archival material collected in histopathology departments all over the world.