[en] Sindbis virus-chronically infected L-cells were established in 14 day-old monolayers by changing the medium every two hours after virus inoculation, and in 24 hour monolayers by changing the medium every 2 or 3 days. Fourteen day-old L-cell monolayers were cured of the Sindbis virus primary infection by changing the medium every two or three days. L-cells produced a small amount of interferon on primary infection with Sindbis virus, and none or undetectable amounts once chronically infected. Chronically infected L-cell monolayers were cured by adding 220 units of interferon per ml to the system. Sindbis virus chronically infected L-cells were resistant to superinfection with homologous virus, and with the heterologous viruses NDV and influenza A PR₈. Doses of 1,000 and 10,000 rads of gamma radiation had no effect on the state of chronicity or on virus production. Spontaneously cured L-cells behaved the same as previously uninfected L-cells. A review of the literature on interferons and their importance in persistent infections is included. 92 references