Abstract: Normal transfer factor (N-TF) and S,w specific transfer factor (S₁₈₀-STF) were extracted by means of disruption-dialysis method from lymphoid tissues of rabbits. The killing and inhibito-ry effects of N-TF and S 180-STF on S o cells were obsevered. The results showed that both N -TF and S₁₈₀-STF could inhibit the DNA synthesis of S₁₈₀ cells but could not kill the tumor cells. The percentage of inhibition correlated with the dose of transfer factor S₁₈₀-STF could also markedly inhibit transplant ability of S o cells.