Progressive enzymic degradation of human urogastrone-epidermal growth factor (URO-EGF) has given a series of fragments shortened at the C-terminus leading to removal of 20% of the structure. These peptides have been evaluated for their ability to bind to polyclonal antiserum and to isolated membrane receptors, to stimulate thymidine uptake by fibroblasts and to inhibit gastric acid secretion in rats. The related molecule human transforming growth factor-alpha, was also assayed and showed similar potency to URO-EGF in all systems. Reduced binding to the receptors of the fragments was paralleled by reduction in both biological activities indicating that this portion of the molecule was concerned entirely with receptor binding. After removal of 11 amino acids from the C-terminus the residue peptide was a full agonist although higher concentrations were necessary.