We report a simple method to obtain internal amino acid sequences from larger proteins electroblotted to polyvinyldifluoride membranes. To demonstrate this method, immunoglobulin heavy and light chains are separated by gel electrophoresis and electroblotted to the membrane. The separated chains, immobilized to the membrane, are cleaved in situ by cyanogen bromide and the resulting fragments are subsequently eluted from the membrane. The fragments are separated by gel electrophoresis, electroblotted and subjected to gas phase microsequence analysis. © 1988 Academic Press, Inc.