The DNA content of 44 giant cell tumors (GCTs) was determined by flow cytometry and simultaneous analysis of cell surface antigens and cellular DNA was performed in two of these GCTs. Two grade III GCTs, one with lung metastases, were all aneuploid. Out of 42 grades I and II GCTs, 16 were aneuploid, statistically, there was no difference in proliferative activity and rate of aneuploidy between grade I and grade II GCTs, but significant difference between primary and local recurrent GCTs existed. After 5 passages in culture the proliferative rate of cells in 13 GCTs increased significantly, the rate of cells bearing GCT associated antigen stepped up and in 2 GCTs tested specially. All these findings demonstrate: (1) the morphological difference of these tumors was not remarkable (2) DNA determinations as an adjunct to conventional histopathologic assessment may provide objective clinically relevant information for prognosis, (3) tumor and non-tumor cells co-exist in GCTs and only tumor cells could survive in long-term culture in vitro.