To investigate the effects of yak transfer factor (TF) on the proliferation of peripheral blood lymphocytes in Tibetan sheep, the effects of lymphocytes, fetal calf serum (FCS) and concanavalin A(ConA) of different concentrations on promoting lymphocyte proliferation were tested by MTT test. Under optimal conditions, the effect of yak TF on peripheral blood lymphocyte proliferation in Tibetan sheep was explored. The results of the experiment demonstrated that, under the combination of lymphocyte, ConA and FCS of 5.4×10⁶ unit/mL, 1 μg/mL and 8% respectively, the best stimulation effect could be achieved. Yak TF only or TF with ConA of 1.5 mg/mL (the best concentration) could also promote lymphocyte proliferation. 3 days after yak TF was injected in each dose group, the proliferation effect of sheep lymphocytes under the stimulation of TF only or TF with ConA became stronger, but the difference was not significant compared with the individual proliferation effect of cell groups (P>0.05). The effect was significantly higher than that of the cell groups at 5-7 days after the injection (P<0.05). The conversion proliferation effect of the 2 mL dose group was better than that of the 1 mL and 3 mL groups, and the difference was significant (P<0.05). Yak TF can effectively enhance the immunity.