Different level chlortetracycline (0,50,150mg/kg) were fed to BALB/c Mice for several weeks. Index of immune organs and proliferative response of mitogen-induced spleen T.B lymphocyte were tested. The changes of tissue in spleen and thymus were also observed with microscope. The results revealed that spleen index in group B(50mg/kg) and T lymphocyte prolifevetive response in B.C groups were higher than those in group A. There were no significant difference in other immune indexes between test groups and control. All above demonstrated that chlortetracyline additive used in promoting animal growth had no inhibition to development of immune organs and T.B lymphocyte function of BALB/c Mice.