Smooth muscle autoantibodies in infectious mononucleosis.

Smooth muscle antibodies (S.M.A.) were found in the sera of 81% of 126 patients over 10 years old with seropositive infectious mononucleosis tested within one month of onset. In 27 patients presenting clinically with infectious mononucleosis but having negative Paul-Bunnell tests the incidence of S.M.A. was 44%. In children 10 years of age or less in these two categories S.M.A. were present in 75% and 25% respectively, while in children of similar age not suspected of having infectious mononucleosis the incidence of S.M.A. was 10%. Among 45 adults with past histories of seropositive infectious mononucleosis more than one year before the incidence of S.M.A. was 33%, in contrast to 14% in 98 subjects with a negative history for infectious mononucleosis.