The hapten-binding myelomas of BALB/C mice provide monoclonal populations of antigen-binding B-cells which are responsive to immunoregulatory effectors induced in the tumor-bearing host. Specific, immunologic regulation of myeloma cell proliferation, differentiation, and immunoglobulin expression demonstrates many of the features which occur in T-cell mediated regulation of non-neoplastic antigen-binding B-cells. Immunoregulation of MOPC-315 cell proliferation and immunoglobulin expression can be induced by: (a) immune response directed to the idiotypic antigens of M315, and (b) carrier-specific presentation of TNP to the cell surface membrane TNP receptor of MOPC-315 cells in carrier-primed syngeneic hosts. The proposed research is aimed at identifying and characterizing the molecular and cellular mechanisms which mediate carrier-specific and idiotype-specific immunoregulation of MOPC-315 growth and differentiation in vivo and in vitro. A major objective is the purification and chemical characterization of the carrier-specific immunoregulatory factor(s). These studies are likely to contribute to a basic understanding of the immunologic regulation of normal and neoplastic B-cells and may have implications for the control of human B-cell neoplasms such as multiple myeloma.