A fibrogenic factor which stimulates collagen production without cell proliferation of rat skin fibroblast cultures was isolated from CCl4-damaged rat liver. (1) The factor was isolated from saline extracts of CCl4-induced fibrotic rat liver and fractionated by Sephadex G-50S gel filtration and DEAE-cellulose chromatography. The original extract produced a 6-fold increase in collagen synthesis and the active factor eluted from gel filtration columns in a region corresponding to 5000 daltons. (2) The active factor was destroyed by heat (57 degrees C, 30 min), phospholipase C digestion, but was insensitive to proteolytic enzymes or phospholipase A. Chemical analysis of the partially purified factor revealed relatively high quantities of phosphorus (3%) and low quantities of protein (13.3%), neutral sugar (1.9%) and uronic acid (4.9%). The possibility of this component being a complex phospholipid containing polypeptide is suggested. (3) Fibrogenic properties of the isolated factor was enhanced by apparent oxidation in air, to a more active, yet insoluble complex. Attempts to solubilize the oxidized product completely destroyed its biological activity.