The release of Kirsten mouse leukemia virus and a Kirsten mouse sarcoma-leukemia virus was studied in synchronous cell cultures. Chronically infected mouse and rat cells were synchronized by a double thymidine block. Virus release into the culture fluids was monitored at 2-hour intervals after release from thymidine inhibition by focus-forming and focus-inducing assays. Maximal virus release coincided with the time of mitosis in both cell systems.