A growth of malignant cell line (K562) by platelet derived growth promoting factor (PDGF) was studied in comparison with that of fibroblast. PDGF was partially purified from platelet lysate by ultracentrifugation and gelfiltration. This factor had a molecular weight of 20, 00030, 000 dalton and was extremely stable to treatments with heat (100°C 2 min), alkli or acid. 3 T 3 fibroblasts at confluent state remained quiescent when they were cultured with PDGF in serum free medium although addition

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... of plasma to this medium promoted these cells to enter into DNA synthesis. On the other hand, malignamt cells went proliferation by stimulation of PDGF alone without any plasma components. This independency from plasma requirement, however, was not unique character to tumor cells but was also observed with 3 T 3 fibroblast at logarhythmic phase. These results indicated that PDGF itself is mitogenic to bring cell cycle into S-phase in tumor cells as well as sparse fibroblasts, and plasma is required for quiescent cells to become competent to synthesis DNA. In addition to this PDGF activity, platelet lysate was found to contain dialysable factor which also promoted proliferation of tumor cells. Physicochemical properties of this factor are now under investigation.