Epidermal growth factor (EGF) was labeled with 125-I by a lactoperoxidase technique. The unlabeled, monoiodinated and diiodinated species were separated by DEAE-cellulose chromatography and found to possess equivalent biological activities. The binding of monoiodinated epidermal growth factor to human fibroblasts was specific in that unrelated polypeptides did not affect the binding reaction. The binding reaction was a saturable process and was time- and temperature-dependent. A Scatchard analysis of the binding data indicated that each cell was capable of binding approximately 100, 000 molecules of 125-I-EGF. The apparent dissociation constant for the binding reaction was calculated to be 2.7 to 4.3 times 10-minus 10 M. Subsequent to the binding of 125-I-EGF to the fibroblasts, the growth factor was degraded by a cell-mediated proteolysis and [125-I]monoiodotyrosine appeared in the medium. The extent of degradation was reduced by the protease inhibitors, tosyl-L-lysine chloromethyl ketone and the benzyl ester of guanidobenzoic acid. Active binding sites of 125-I-egf appeared to be present in some but not all cell types. These results demonstrated that cells derived from a number of species (human, mouse, rat, and chick) possessed receptors that interacted with this mouse-derived growth factor.