Monoclonal antibodies (MAbs) to nuclear antigens are increasingly used as tools to obtain valuable information concerning the proliferative characteristics of various types of cancer. Prerequisite for the application of these MAbs in surgical pathology is establishment of the level of expression and/or cellular distribution of the antigens in relation to distinct cell-cycle compartments. In this study the topologic distribution of proliferating cell nuclear antigen (PCNA), an auxiliary protein of DNA polymerase delta, as recognized by human autoantiserum (AK) and two recently developed MAbs (19A2 and 19F4), was evaluated. Using cultured human cancer cells as a model system, and providing optimal fixation/permeation procedures are applied, these antibodies display a high affinity for PCNA bound to nuclear replicon clusters, resulting in distinct granular staining patterns. A more diffuse nucleoplasmic PCNA staining was mainly restricted to non-S-phase cells; in methanol-fixed cells, staining intensity of this form relative to the replicon-bound form appeared higher after staining with 19A2 than with 19F4 or AK. Comparing PCNA expression (detected with 19A2) with the expression of the Ki-67 antigen, PCNA-negative cells are also Ki-67 negative. In MCF-7 human breast cancer cells treated with 10(-6) mol/l (molar) tamoxifen, the fraction of nuclei showing replication patterns decreased from 42% to 8% within 8 days, but PCNA and Ki-67 antigens remained detectable in most cells during this interval, indicating a relatively slow decrease of antigen expression in cells that have entered a quiescent state. Treatment of MCF-7 cells with 10(-6) mol/l methotrexate resulted in a rapid accumulation of cells with an early S-phase DNA content; PCNA replication patterns showing a frequency distribution reflecting this DNA content were observed up to 48 hours after treatment. This indicates that the presence of replication patterns as visualized with anti-PCNAs is not a measure of replicative activity per se. It is concluded that, providing nuclear non-S-phase PCNA staining is faint relative to staining of replicon clusters, anti-PCNA antibodies may be excellent markers to detect in situ cells with S-phase DNA contents.