Using monoclonal antibodies, we demonstrate that the phosphoprotein of measles virus and a protein of herpes simplex virus type 1 crossreact with an intermediate filament protein of human cells. This intermediate filament protein, probably vimentin, has a molecular weight of 52,000, whereas the molecular weights of the measles viral phosphoprotein and the herpes virus protein are 70,000 and 146,000, respectively. Crossreactivity was shown by immunofluorescent staining of infected and uninfected cells and by immunoblotting. The monoclonal antibody against measles virus phosphoprotein did not react with herpes simplex virus protein and vice versa, indicating that these monoclonal antibodies recognize different antigenic determinants on the intermediate filament molecule. The significance of these results in explaining the appearance of autoantibodies during virus infections in humans is discussed.