The 2-amino alcohol structure -CH(NH2)CH(OH)- exists in proteins and peptides in N-terminal Ser or Thr and in hydroxylysine. Its very rapid oxidation by periodate at pH 7 generates an aldehyde in the peptide and is the first step in a method for site-directed labeling with biotin or a fluorescent reporter. The modifying group is a hydrazide, RCONHNH2, which reacts with the new aldehyde to form a hydrazone-peptide conjugate, RCONHN = CH-peptide. Experiments with two synthetic peptides, Ser-Ile-Gly-Ser-Leu-Ala-Lys and Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly, and with recombinant murine interleukin-1 alpha (an 18-kDa cytokine with N-terminal Ser) demonstrated this method of peptide tagging. The use of a low molar ratio of periodate to peptide minimized the potential for side reactions during the oxidation, and the desired oxidation was rapid and highly specific. The hydrazones formed were stable at pH 6-8 for at least 12 h at 22 degrees C, but were labile at more acidic pH values. Potential uses of this method include the attachment of biotin, reporter groups, metal chelating groups, imaging agents, and cytotoxic drugs to peptides.