The genetic approach to the problem of cellular growth control is limited by the availability of recessive mutations in cell lines which are capable of growth control in vitro. The CHO cell line has yielded many recessive mutations including, for example, tsH1, a temperature sensitive leucyl-tRNA synthetase mutant, which under non-permissive conditions rapidly shuts down protein synthesis and generates uncharged tRNA. Both CHO and tsH1 are transformed, however, and do not respond to environmental stimuli with the coordinated regulation of macromolecular processes observed in normal diploid fibroblasts. We describe here the isolation and characterization of growth control revertants obtained from both CHOwt and tsH1. The best of these GRC+L-73, isolated from tsH1, had 20 chromosomes, one less than tsH1, had normal fibroblastic morphology, would not grow in suspension, required high serum concentrations for growth, grew to relatively low cell densities at saturation in monolayer culture and showed a stationary phase characterized by arrest in a G1-like state with maintenance of high viability for several weeks. It is expected that this line as well as a ts revertant GRC+LR-73 will greatly facilitate the genetic investigation of growth control and, in particular, will help to elucidate the role of uncharged tRNA in the regulation of macromolecular synthesis in mammalian cells.