Abstract Certain epidemiologic features of infectious mononucleosis are indicated by antibody patterns to the EB or herpes-like virus of Burkitt lymphoma. The absence of antibody correlated well with susceptibility: of 268 entering college students whose serums lacked EBV antibody, infectious mononucleosis developed in 15 per cent; in 94 whose serums already contained antibody, none had clinical disease. EBV antibody also correlated well with heterophil antibody; EBV antibody was present in the serums of all 135 patients who had heterophil-antibody-positive infectious mononucleosis and also in six with clinically and hematologically typical cases whose serums were persistently heterophil-antibody negative, suggesting that EB virus may be associated with both forms of illness. In a third group of patients clinical features resembled infectious mononucleosis, but the serums contained neither heterophil nor EBV antibodies. At present, EB virus is strongly implicated as a cause of infectious mononucleosis.