Sera from 2,829 participants in a population health survey were tested for the presence of a heterophile antibody, detectable by immunofluorescence. The antibody was found in 9.8% of the sera from women and in 12.4% of the sera from men. The age-specific prevalence of the antibody was relatively constant in women but rose steeply in men. The antibody occurred more frequently in individuals with a history of blood transfusion and in multiparous females. It is suggested that the antibody may result from immunization by a ubiquitous agent which can be transmitted by blood transfusion.