The effects of tumor.necrosis factor-α (TNF) on protein metabolism and cell-cycle kinetics were investigated in malignant tumor. Sprague-Dawley rats, subcutaneously inoculated with Walker 256 carcinosarcoma, were injected intraperituneally with recombinant human TNF at a dose of 4.75×10⁶ U/kg for 3 consecutive days. Tumor protein metabolism and cell-cycle kinetics were analyzed. The results showed a significant decrease in tumor volume and weight in comparison with control. TNF resulted in significant decrease in tumor protein fractional synthesis rate, protein synthesis and fractional growth rate, but no change of tumor protein fractional degradation rate. TNF also resulted in remarkable decline in labelling index and G1 phase increase of tumor cells, 6 hours after bromodeoxynridine injection, by cytometry. The results indicated that TNF inhibits tumor growth as a result of decreases in tumor cell DNA and protein syntheses.