Greenway and his associates reported in 1959 that they failed to propagate Junín virus in monkey kidney tissue cultures as well as KB and HeLa cells. Nevertheless, it was decided to attempt cultivation of Junín and Tacaribe viruses in HeLa cells according to methods that have been successfully applied in The Rockefeller Foundation Virus Laboratories to the study of arboviruses. Drs. Mettler, Casals and Rapp shared in some aspects of these experiments. The present studies were done with three strains of Junín virus, namely, the F, R5 and XJ strains, and with Tacaribe virus, strain TR 11573. All four virus strains were mouse-adapted. HeLa cell cultures were inoculated with ten-fold dilutions of infected newborn mouse brain suspensions and examined daily for development of cytopathic effects. The morphological changes induced by the replication of both Junín and Tacaribe viruses in HeLa cells occurred with all four strains in the first passage, between 5 to 7 days after inoculation.