To elucidate the mechanisms of autoantibody induction in infectious mononucleosis (IM), we have studied sera from 35 patients with IM with enzyme-linked immunosorbent assays using purified antigens. In the IM group 37% had IgM antibodies to cardiolipin above the normal range (mean plus 2 standard deviations of control sera). Significantly elevated frequencies of antibodies to actin (26%) and cytoskeletal antigens (97% versus 29% in normal sera) were also found, but levels of IgM rheumatoid factors, IgM antibodies to single-stranded DNA and antibodies to ribonucleoproteins (nRNP/Sm, Sm and La) were normal. Affinity purified anti-cardiolipin antibodies reacted with the cell membrane of transformed lymphocytes but not with resting cells, suggesting that cell activation was required for the expression of antigenic epitopes. Our data suggest that the autoantibody response in IM is restricted to two classes of autoantigens: cytoskeletal and cell membrane antigens. The appearance of antigenic epitopes on EBV-transformed lymphocytes could be a mechanism for the generation of anti-cardiolipin antibodies in infectious mononucleosis. Similar mechanisms could operate in autoimmune rheumatic disease.