58 MCA Cl 16 is an oncogenic methylocholanthrene-transformed variant of the non-transformed mouse embryo fibroblast cell line, C3H/10T1/2 Cl 8. Using two different protocols, they have isolated six temperature-sensitive mutants from N-methyl-N' - nitro N-nitrosoguanidine treated cultures of 58 MCA Cl 16. C3H/10T1/2 Cl 8, 58 MCA Cl 16 and the six mutant lines were characterized with respect to several properties associated with the transformed state: morphology, saturation density, anchorage independence, cell surface morphology and growth in medium containing 1% fetal calf serum. In general, C3H/10T1/2 cells behaved as non-transformed, whether grown at 33 C or 39.5 C. The transformed parental line and all six mutants behave as transformed cells at 33 C. At 39.5 C only the parental transformed line retained the transformed phenotype. Three of the mutants revert towards non-transformed behavior at 39.5 C for all of the properties rested. The remaining mutants are temperature-sensitive for some, but not all, transformed characteristics. Thus, while the expression of these transformed properties is sometimes coupled, the authors have been able to dissociate the expression of traits such as saturation density, anchorage independence and transformed morphology from each other. These mutants should prove to be valuable tools in the study of the mechanisms which underly the expression of the chemically-induced transformed state.