We have established and characterized five new monoclonal antibodies (mAbs) which specifically immunoprecipitate the human interleukin-2 receptor beta chain (IL-2R beta). One of them, TU30, recognizes the intracytoplasmic 'serine-rich region' of IL-2R beta that is critical for IL-2 signal transduction. The others, TU12, TU21, TU23 and TU25, completely inhibit IL-2 binding, as does the previously characterized TU27. However, reciprocal binding competition assays show that the epitopes recognized by the individual mAbs are different from each other. The mAbs inhibit the growth of IL-2-dependent cells. The magnitude of their inhibitory effects is dependent on not only the affinities of the mAbs for IL-2R beta but also upon the number of IL-2R alpha subunits expressed on IL-2-dependent cells. These mAbs should be useful in studying the structure and function of the IL-2R.