The relationship between the position of chick embryo fibroblasts in the mitotic cycle and their susceptibility to infection by B-RSV (RAVI) was studied in synchronized cultures. The pattern of virus growth was similar in cultures infected at various times during the mitotic cycle. Notably, first virions were released after comparable delays in cultures infected before, during or after the S phase, and the virus production was identical when measured up to 48 h after infection. Latent periods extended from 8 to 15 h according to the experiment. Identical results were obtained with both calf sera used to induce mitoses. These results show that the S phase of cellular DNA synthesis is not required at some early stage of the viral cycle and suggest that there is no obligatory relationship between RSV production and the cell cycle.