The 106-amino acid sequence motifs of spectrin have been suggested to fold into stable structural domains, consisting mostly of coiled coils of triple helices. With the advent of molecular biology and biophysical techniques, structural studies of these spectrin 106-amino acid structural domains became approachable. However, one of the difficulties in such an approach is determination of the correct phasing of the structural domains, which may or may not coincide with the phasing of the sequence motifs. Proper identification of the domain phasing is vital to the construction of stable spectrin domains for molecular studies. A previously published phasing shift for Drosophila alpha-spectrin indicated a downstream phase-shift of 26 amino acids for the structural domain (Winograd, E., Hume, D., and Branton, D. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 10788-10791). Using this phase-shift, we prepared a recombinant spectrin peptide with the sequence from residue 49 to residue 155 of human erythrocyte alpha-spectrin and found this peptide to be unstable relative to other peptides that we prepared. Using several other recombinant alpha-spectrin peptides and following the protease digestion approach, we digested spectrin peptides with elastase and chymotrypsin and analyzed the amino acid sequence of the digestive products. We provide the first experimental evidence in identifying the first amino acid residue of the first spectrin domain in human erythrocyte alpha-spectrin as residue 52 (Ser).