The duration of the latent period of RSV was studied in unsynchronized chicken cells using two methods. First, RAVI-producing cells were infected with Schmidt-Ruppin strain of Rous sarcoma virus (SR-RSV; subgroup D), and new virions of SR-RSV (subgroup A) produced after phenotypic mixing with RAVI were neutralized with an anti-serum to RAVI. Secondly, uninfected chicken cells were infected with SR-RSV (D) in the presence of either arabinosylcytosine, actinomycin D or cycloheximide, to inhibit the virus production. In both methods, the early decrease of the virus titer showed that new virions were produced as early as 45 min after the beginning of the infection. Earlier times were not explored.