A persistently infected cell culture was established from Vero cells surviving primary infection with wild-type Junin virus (JVwt). The cells were refractory to superinfection by both Junin virus and the related Tacaribe virus. Replication of virus released from the persistently infected cells (JVpι) was inhibited at 39°. JVpι did not interfere with JVwt at 37°, and it was inactivated at 37, 40, 43, 46 and 50° much more rapidly than was JVwt.