Blood sera of 300 adult hospital cases were tested for agglutinins for sheep erythrocytes. In this control group the agglutinin titer never exceeded 1:16. 13 of 15 accepted cases of mononucleosis, studied in the acute stages, showed an increased titer of agglutinins for sheep erythrocytes, attaining dilutions as high as 1:4,096. Except in these subjects, titers above normal levels were encountered only under the following circumstances: (1) in individuals who had received injections of horse serum; (2) in 1 patient whose serum contained agglutinins for several bacterial antigens and who probably had the mononucleosis; and (3) in 1 patient with purpura hemorrhagica who presented scant evidence of a concomitant mononucleosis. Blood serum of several cases of infectious mononucleosis contained agglutinins for a number of bacterial antigens; in 1 case, the Wassermann reaction became temporarily positive.