The prevalence of encephalomyocarditis virus (EMC) antibodies among selected human populations in various regions of the world was determined by the plaque reduction neutralization method. Antibody rates among children ranged from 1.0 to 33.9%, while those among adults varied from 3.2 to 50.6%. No differences between sexes were found in the frequency of EMC infection. The pattern of age-specific antibody rates observed among the study populations suggests that EMC infection occurs primarily during childhood. There appeared to be no association between the presence of EMC antibodies and potential exposure to rats. Sera from diabetic, suspected encephalitis, and myocarditis patients were also examined for EMC neutralizing antibodies. The prevalence of antibodies among these groups was not significantly different from that of control populations in the same geographic regions. No association was demonstrated between EMC infection and these three diseases. The results of this study indicate that EMC infection in man is fairly common but that most human cases are probably asymptomatic and/or unrecognized.