The proliferative activity in 70 cases of soft-tissue tumours was estimated immunohistochemically using the monoclonal antibody PC-10, which recognizes proliferating-cell nuclear antigen (PCNA) in paraffin sections. The PCNA index (i.e. the percentage of positive neoplastic nuclei) and to a lesser degree the PCNA count (i.e. the number of positive neoplastic nuclei per ten high-power fields) positively correlated with the malignancy grade (PCNA index: P < 0.001; PCNA count: P < 0.01). However, the range of values of PCNA index and PCNA count was similar between benign and grade I tumours. A statistically significant positive correlation was also established between PCNA index and PCNA count on the one hand and mitotic count on the other, but the correlation coefficient was low (r = 0.351, P < 0.01, and r = 0.290, P < 0.05 respectively). These results indicate that PCNA immunostaining may successfully be used as an adjunct to the conventional histopathological parameters in assessing the malignancy grade in soft-tissue tumours although it is of limited value in distinguishing between benign and grade I tumours.