Two monoclonal antibodies (McAbs), HI160 and HI166, recognize different epitopes of the CD71 antigen. Their biological properties have been studied, and the results demonstrated that both had down-regulation effects. HI160 McAb not only inhibited the activation and proliferation of T and B lymphocytes but also the growth of CFU-GM. HI166 McAb only inhibited the activation and proliferation of T lymphocytes. The differences in the biological properties of the two CD71 McAbs might be due to the different epitopes recognized by them. The application value of these McAbs is discussed.