Factors stimulating cell proliferation were extracted from the supernatant of chick embryo carcases and adult chicken leg muscles of the same strain. Partial purification was done, and the mechanism of stimulation (without supplements) and physicochemical properties were studied. 1). There were at least two or more stimulating factors for cell proliferation of chick embryo fibroblasts in the supernatant of embryo carcases and adult leg muscles of chickens. 2). The embryonic stimulating factors were several times more active than the muscular ones. 3). One of these stimulants was basic and had a low molecular weight protein-like component. 4). One of the partially purified stimulants showed its strongest activity on DNA synthesis in a chick embryo fibroblast system.