Interleukin-2 (IL-2), the main lymphokine produced by activated CD4+ T-helper lymphocytes, is a potent T-cell growth regulator. In addition to this activity, IL-2 was found to decrease the proliferation of 3T3 fibroblasts in time- and dose-dependent manner. Thus 3T3 cells were seeded overnight at a density of 5,000 cells/well, and treated with or without various concentrations of IL2 for 48h, then 3T3 cells were observed by inverted microscope and cell viability was measured by Cell Counting Kit-8 (CCK-8). Briefly, 10μL of CCK-8 solution was added to each well of the plate, then measure the absorbance at 450nm using a microplate reader after incubating the plate for 1-4 hours at 37oC. The inhibitory effect of IL-2 on cell proliferation of 3T3 cells observed by inverted microscope was shown in Figure 1.