Proliferating cell nuclear antigen (PCNA/Cyclin) is a 36-kD protein that is present in cycling cells but not in resting cells, and therefore represents a marker of tumor proliferation. Application of anti-PCNA/Cyclin monoclonal antibodies has shown that this protein is localized to the nucleus of cycling cells, with the exception of cells in mitosis, which demonstrate faint cytoplasmic reactivity. Recently, Benjamin and Gown found that Reed-Sternberg cells and variants show nuclear and cytoplasmic staining with anti-PCNA/Cyclin antibody 19A2, and suggested that this feature may be useful in distinguishing Hodgkin's disease from other tumors. This report describes the reactivity of 42 workshop cases that were stained with anti-PCNA/Cyclin antibodies 19A2 and/or PC10. Thirty-three (79%) of the 42 cases showed adequate reactivity to allow for interpretation of staining localization. In the group of reactive cases, 26 (79%) showed nuclear and cytoplasmic staining. The localization of PCNA/Cyclin was compared with the consensus diagnosis in each case. Eighty percent of cases classified as Hodgkin's disease, 67% of cases classified as non-Hodgkin's lymphoma, and 100% of unresolved cases showed both nuclear and cytoplasmic staining. The incidence of cytoplasmic PCNA/Cyclin was not different between Hodgkin's disease and non-Hodgkin's lymphoma in this study.