This is a proposal to analyze murine T lymphocytes that have surface membrane Fc receptors. We recently discovered (J. I. 123:1110, 1979) that BALB/c mice with subcutaneous IgA plasmacytomas develop very large numbers of circulating, 0-bearing lymphocytes that express surface membrane receptors (FcR) for Fc determinants of IgA. We propose to extend these studies to: i) determine the generality of elevated FcR+ lymphocytes in myeloma, ii) determine whether the heavy chain isotype of the myeloma protein dictates the isotypic-specificity of the lymphocyte Fc receptor, iii) characterize further the origin and mechanism of induction of FcR+ lymphocytes in myeloma, iv) conduct structural and binding analyses of the purified Fc receptors, and v) assess the immunoregulatory properties of the FcR+ lymphocytes by analyzing their ability to influence the neoplastic B cells in myeloma as well as non-neoplastic B cells induced with conventional immunogens and mitogens. The information generated in these studies is likely: i) to provide insight into the structural basis of immunoglobulin binding to FcR+ lymphocytes, ii) to better define the immunoregulatory functions of FcR+ T cells, and iii) to contribute further to our understanding of the immunology of myeloma.