We have recently described a human IgM monoclonal antibody (mAb), reactive with both self antigens, i.e., cytoskeleton filaments and smooth muscle, and Epstein-Barr virus (EBV)-induced nuclear antigen (EBNA), produced by EBV-transformed B lymphocytes isolated from a patient with infectious mononucleosis (IM).In order to achieve higher antibody secretion in culture supernatant, the mAb-producer cells were fused with ouabain-resistant mouse myeloma cells and a stable human-mouse heterohybrid, coded HY 5488, producing up to 80 //g/ml IgM mAb, was isolated after 4 cloning procedures. Purified HY 5844 mAb was used to immunize mice for the production of a murine anti-idiotypic mAb, which was used to probe the expression of the idiotope of HY 5488 mAb (Id 5488) in sera of IM patients and normal controls by ELISA.It was found that Id 5488 is expressed both in IM patients and normal controls, and that Id 5488 expression is significantly higher in IM patients' sera; furthermore, in IM sera a statistically significant correlation between Id 5488 expression and anti-cytoskeleton and anti-smooth muscle autoantibodies was found.It is suggested that at least part of EBV-induced IgM autoantibodies appearing during IM are secreted by B lymphocytes programmed to the production of“natural antibodies”bearing Id 5488-like idiotopes.