Ki-67 is a monoclonal antibody to a nuclear antigen present in cycling human cells but not in resting cells. The authors have performed immunoperoxidase on non-Hodgkin's lymphomas using Ki-67 antibody in order to correlate proliferation rates with tumor grade and type, and compare Ki-67 staining with S-phase content as determined by flow cytometry. Ki-67 staining of 109 sections was quantitated using a digital image analysis system (CAS 100). There was a significant difference among mean overall Ki-67 staining values in Working Formulation low (13.7%), intermediate (42.6%), and high grade tumors (57.9%, P less than 0.00001). The level of significance improved when a revised grading system was formulated based on proliferative activity, with the inclusion of diffuse large cell lymphomas in the high grade category. Within nodular and a few diffuse lymphomas, there were well-defined proliferation centers in which Ki-67 staining showed no correlation with grade. Flow cytometric DNA determination was performed on 74 specimens, and there was a positive correlation between Ki-67 positivity and S phase content (r = 0.66). It is concluded that Ki-67 staining of tissue sections is an alternative to flow cytometric quantitation of cell cycle activity in lymphomas, and provides the advantage of revealing histologic patterns of proliferation. By including G1 phase cells, Ki-67 staining allows a more complete determination of total cell cycle activity in lymphomas.