Immunoglobulin was obtained from the tumor cells of a patient with nodular lymphoma by hybridization to mouse myeloma cells. The human immunoglobulin secreted by these hybridoma cells was used as an immunogen to make murine monoclonal antibodies. Antibodies specific for idiotype, mu heavy chain and lambda light chain, were produced. One anti-idiotype antibody was used to document that idiotype-positive cells and low levels of 19S IgM idiotype were present in the patient's blood. The levels of each were found to correlate with the patient's disease activity. The monoclonal anti-idiotype was effective in eliminating idiotype-positive cells in vitro by solid phase absorption or by complement-mediated cytotoxicity. The anti-idiotype was also used to analyze the host's immunologic response to his own tumor idiotype. There was neither a detectable anti-idiotype antibody response produced in vivo nor a detectable population of T cells that expressed idiotype of could bind idiotype.