Foci of Rous cells produced by various Rous sarcoma virus (RSV) strains stain with methylene blue and can be distinguished from the background of unstained density-inhibited normal cells. RSV-infected cultures also display areas of stainable cells which are not morphologically transformed. These are not seen in uninfected controls and can be more numerous than the foci. The total number of stainable areas (transformed and untransformed) is proportional to the virus input as is the focus number. Using the virus FU-19, a mutant of SR-RSV thermosensitive for transformation, it has been shown that, after shifting infected cultures from the permissive (37°) to the restrictive (41°) temperature, foci of Rous cells can lose the transformed phenotype but remain stainable for some time. Following the reverse shift (from 41° to 37°), foci can become stainable before acquiring the transformed phenotype. Hence, areas of untransformed stainable cells presumably correspond to latent foci of virus-infected cells and represent a first step in transformation.