Relevant prognostic information can be added to the histological classification of non-Hodgkin's lymphoma (NHL) by the determination of the growth fraction. In unfixed fresh tissue samples the Ki-67 antigen has proven its utility as an operational marker of proliferative activity in NHL. We have generated a monoclonal antibody directed against a formalin-resistant epitope of the Ki-67 antigen, designated Ki-S5. The immunoreactivity of Ki-S5 is confined to the nuclei of proliferating cells and, unlike Ki-67, no cross-reactivity with cytoplasmic antigens of epithelial cells occurs. In 100 cases of different NHL types, parallel staining of Ki-67 and Ki-S5 antigen yielded almost identical results in fresh tissues and fixed tissues, respectively (P < 0.00001; r = 0.95). In some cases, the Ki-S5 labeling index exceeded that of Ki-67, most probably due to a partial degradation of the epitope recognized by Ki-67 in frozen stored tissue samples. Significant differences in the median Ki-S5 labeling index could be demonstrated between high grade (> 50%) and low grade malignant NHL (< or = 30%), although the proliferative activity varied to a considerable range within identical histological categories (P < 0.001). We conclude that Ki-S5 provides a reliable means to assess proliferation in paraffin-embedded NHL specimens. Retrospective studies relating the proliferative activity to clinical outcome are thus rendered possible using archival routinely processed tissue samples.