The expression of the three catalytic subunits of protein phosphatase (PP) type 1 and 2A, PP1 alpha, PP1 gamma 1, and PP2AC, was examined in osteogenic tumors and soft tissue tumors by immunohistochemical analysis. The percentage of cells stained positively with antiserum against PP1 catalytic subunit isoform PP1 gamma 1, was significantly higher in malignant osteogenic tumors (chondrosarcoma, osteosarcoma, and Ewing's sarcoma) and in malignant soft tissue tumors (liposarcoma and malignant fibrous histiocytoma [M.F.H.]) than in benign tumors (osteochondroma, osteoblastoma, ossifying fibroma, enchondroma and lipoma). Furthermore, the malignant tumor lesions showed a markedly high number of cells in the S-phase fraction of the cell cycle, as compared to benign tumors. These results suggest that PP1 gamma 1 is involved in the accelerated growth of malignant tumor cells.