A method for identification of a particular kind of peptide in a mixture by identical electrophoretic steps, the second at a 908 angle to the first, with a chemical modification introduced between the steps. An example is the identification of tyrosine-bearing peptides in a mixture of peptides by treatment of the products of paper electrophoresis with iodine vapour to iodinate the tyrosine residues before the second electrophoresis. Iodotyrosine peptides are then identified as those that deviate from the diagonal formed by all the other peptides when they are visualised by, for instance, the ninhydrin reaction.