The presence of particles similar in morphology to avian sarcoma leukosis viruses has been confirmed in 17 cloned lines of nonproducing (NP) cells which have been infected with Rous sarcoma virus. A quantitative study with the electron microscope indicated that, on the average, the number of virus particles in NP cells was about 1.5% of what is usually found in cells infected with Rous-associated virus and about 3% of what is found in Rous virus-producing cells. There was a significant variation in the number of particles among the NP lines examined. © 1968.