Abstract: A new amino derivative, N.sup..alpha. -tert-butoxycarbonyl-N.sup..epsilon. -(N-bromoacetyl-.beta.-alanyl)-L-lysine (BBAL), has been synthesized as a reagent to be used in solid-phase peptide synthesis for introducing a side-chain bromoacetyl group at any desired position in a peptide sequence. The bromoacetyl group subsequently serves as a sulfhydryl-selective cross-linking function for the preparation of cyclic peptides, peptide conjugates and polymers. BBAL residues are stable to final HF deprotection/cleavage. BBAL peptides can be directly coupled to other molecules or surfaces which possess free sulfhydryl groups by forming stable thioether linkages. Peptides containing both BBAL and cysteine residues can be self-coupled to produce either cyclic molecules or linear peptide polymers. Such peptide derivatives are useful in preparing potential peptide immunogens, vaccines and therapeutics, and for substances such as peptides linked to polymers, plastics, enamels and ceramics.