Proliferative activity of soft tissue sarcomas (STS) in 34 cases was estimated by immunohistochemical procedures (avidin-biotin complex [ABC] method) with monoclonal antibody Ki-67 which reacts with a nuclear antigen expressed in all phases of cell cycle except G0. In 20 of 34 cases (59%), varying numbers of Ki-67-positive tumor cells were detected with a range from 5 to 382 per 10 high power fields (HPF) (mean 57.2/10 HPF). Ki-67 index (the number of Ki-67-positive tumor cells/10 HPF) positively correlated with mitotic count (r = 0.428, P less than 0.02), cellularity (r = 0.447, P less than 0.01), and histologic grade (r = 0.473, P less than 0.01). The Ki-67 low index group (less than 50/10 HPF) showed more favorable prognosis than the high index group (more than 50/10 HPF) (P less than 0.005). Three cases with low mitotic count and unfavorable prognosis were proved to be the Ki-67 high index group (142-382/10 HPF). These results indicated that reactivity of tumor cells for Ki-67 is a useful prognostic marker in the patients with STS, and might be used as one of the histologic factors for the grading of STS.