Maleylation of the lysyl residues allows specific cleavage by trypsin at the arginyl bonds. The maleyl amino group is very stable at alkaline pH and is therefore unlikely to hydrolyze during prolonged enzymatic digestions at alkaline pH, a hazard inherent in alternative reversible blocking groups such as trifluoroacetyl-, or tetraftuorosuecinyl-, which are alkali labile. Cleavage at some arginyl residues in maleylated proteins seems to occur much more slowly than at others, probably because of inhibition by adjacent negative charges, and this kinetic specificity can be an advantage in obtaining large peptides for sequence studies. The maleyl group can, of course, be removed from each peptide by acid incubation to allow further tryptic cleavage at lysine residues. The charge change of +2 consequent on such unblocking is the basis of a diagonal electrophoretic separation which allows N-terminal and lysine-containing peptides to be specifically purified from enzymatic digests.