An evaluation of reciprocal indirect immunoíiuorescent antibody titers for at least one representative of each known arenavirus serotype leads to the conclusion that the Old World arenaviruses are more closely related to each other than to other members of the group. These relationships are particularly evident in evaluations of human convalescent sera. New World arenaviruses cross-react strongly with each other in most instances but only weakly with Old World arenaviruses.