Mouse embryo fibroblasts of C3H10T1/2 cell line were selected in the medium with a low serum content. The frequency of clone occurrence was about 1.10(-5) (for 0.5% serum). The treatment of cells by N-methyl-N'-nitrosoguanidine significantly increased the frequency of occurrence of these clones. The obtained spontaneous variants were assayed for stability of growth characteristics. The phenotypic analysis of clones with low serum growth dependence discovered at least two cell types differing in some morphological and growth characteristics. The former did not differ from the parental line C3H10T1/2 in any phenotype characteristics but their serum growth dependence; whereas, the latter were poorly spread over the substrate and showed an enhanced saturation density and a decreased population doubling time. These cells also differed in their growth dependence on the homologous conditioned medium.