A BHK/21 cell line persistently infected by an arenavirus is described. During four consecutive passages, 30–45% of the cells showed granular cytoplasmic antigen by indirect immunofluorescence, employing both Argentine hemorrhagic fever convalescent sera and sera from animals immunized with Junin virus. Virus isolated from the cells killed suckling mice but not adult mice and protected guinea pigs against further challenge with the virulent prototype strain of Junin virus. Neutralization tests showed that the virus isolated from the cells was neutralized by anti-Junin virus antisera. The usefulness of this cell line in rapid immuno-fluorescent serological procedures is describe