Callitrichid hepatitis (CH) is a highly fatal, emerging arenavirus disease of captive South American marmosets and tamarins (Callitrichidae), including the endangered golden lion tamarin. A common-source outbreak of CH in golden lion tamarins and pygmy marmosets at a US zoo resulted from a single feeding of the primates with newborn mice in apparently infected with lymphocytic choriomeningitis virus (LCMV). Isolates from livers of mice and primates were related to isolates from previous CH outbreaks and to laboratory strains of LCMV by serology and nucleic acid hybridization, and 2 surviving animals developed antibody to other LCMVCH isolates and to laboratory strains of LCMV. Thus, LCMV, an arenavirus prevalent in wild mice in the US, can cause sporadic fatal hepatic disease in primates. Exposure of humans to wild or laboratory mice or to marmosets and tamarins that are infected with wild-type strains of LCMV poses the danger of serious disease.