Interleukin 4 (IL4) is a cytokine that induces differentiation of naive helper T cells (Th0 cells) to Th2 cells.It plays vital biological roles, including the stimulation of activated B-cell and T-cell proliferation, and the differentiation of B cells and monocyte. It is a key regulator in humoral and adaptive immunity. IL4 induces B-cell class switching to IgE, and up-regulates MHC class II production. To test the effect of sheep IL4 on cell proliferation, TF-1 cells were seeded into triplicate wells of 96-well plates and allowed to attach, replaced with various concentrations of recombinant sheep IL4. After incubated for 72h, cells were observed by inverted microscope and cell proliferation was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10 μl of CCK-8 solution was added to each well of the plate, then the absorbance at 450 nm was measured using a microplate reader after incubating the plate for 1 hours at 37 ℃. Cell viability was assessed by CCK-8 assay after incubation with recombinant sheep IL4 for 72h. The ED50 of recombinant sheep IL4 is 0.077 μg/ml.