The authors report about five previously selected cell lines of transformed mouse and rat fibroblasts, capable of growing in the medium lacking exogenous proteins. In contrast to cells of the parental lines, the cells under investigation have the ability to semi-suspension growth pattern and demonstrate an increasing sensitivity to damaging action of trypsin. The lowering of the autological conditioned medium (CM) concentration below 10-15%, as well as the reducing of the initial population density to 0.2 x 10(4)-0.4 x 10(5) cells per 1 cm2, resulted in inhibition of the serum-free cell proliferation. The feeder cultures of the serum-free cells immobilized by X-ray are able to stimulate proliferation of the intact autological cells in medium with semi-liquid agar in the presence of serum. CMs of the serum-free cell cultures stimulated the DNA synthesis in stationary cultures of NIH 3T3 cells in the absence of serum. The loss of dependence on serum was accompanied by a considerable broadening of the metionine-containing protein spectrum in the culture medium of the transformed rat embryo fibroblasts. In addition, it was demonstrated that selection for independence on exogenous proteins was accompanied by the loss of oncogenicity in both sensitive and resistant to cytotoxic effect of ethidium bromide serum-free cell lines, derived from L929 cell line. On the contrary, the serum-free variants from spontaneously transformed rat embryo fibroblasts as well as from C3H 10T1/2 cell line exhibit an enhanced oncogenicity as compared with the parental cells.(ABSTRACT TRUNCATED AT 250 WORDS)