Arenaviruses are known to show antigenic relationships in the complement-fixation (CF) and fluorescent antibody tests but not in the neutralization test. The humoral response to some of the arenaviruses is characterized by a dissociation between the antibodies determined by the CF test and those determined by the neutralization test and also by its late appearance following natural infection. Investigations are reported showing that, as regards the CF test, Junin, Machupo, Amapari, and Tacaribe viruses are closely related, while LCM and Lassa viruses are distantly related to each other and to the remaining viruses in the group. Studies by agar gel diffusion and precipitation have shown complete specificity except among viruses that are very closely related in the CF test. Observations with sera from persons infected with Lassa fever virus show that CF antibodies rarely appear before the 18th day after onset; they can persist, with diminishing titres, for up to 6 years.