The addition of serum to the skin organ culture medium enhances the general survival of the explant [1, 2, 10, 13] and, in particular, greatly stimulates the lateral migration of the epidermal cells [3, 5, 9 1 1 , 13]. These two effects have recently been shown to be unrelated [3, 13]. While the influence of serum on the general viability of a skin explant is probably unspecific in nature [13], the cell migrationstimulating effect has been ascribed to a specific factor in serum [7, 8, 10, 11, 13]. This factor is species non-dependent, it is probably glycoprotein in nature, it is needed continuously during incubation, and its effect is independent of the mitotic status of the epidermis [4, 6, 8 -11] . Unfortunately, this migration-stimulating factor has not yet been biochemically purified, and some controversy still exists as to its physicochemical characteristics. Purification efforts have been hampered by the fact that a considerable part of the activity is precipitated during dialysis [3, 7], by the inactivation of the factor during affinity chromatography [8], and by inconsistency in results when applying ammonium sulphate precipitation or gel column chromatography [7]. In the present study, membrane ultrafiltration is shown to be a convenient primary step in the purification of the epidermal cell migration-stimulating factor, and data are given on the physicochemical stability of this factor.