A new procedure is described for sequencing of peptide mixtures by a combination of Edman degradation and field-desorption mass spectrometry. The procedure involves measurement of the mass values of quasi-molecular ions ([M + H]+) of constituent peptides in the mixture and their fragments degraded by the Edman method. Calculation of all the possible mass differences of the mass values before and after degradation and identification of the phenylthiohydantoins released reveal the N-terminal amino acids of individual peptides in the mixture. Repetition of these operations gives the amino acid sequences of individual peptides in the mixture. As typical examples, the sequencing of peptide mixtures prepared by proteolytic digestion of glucagon are described. In addition, a computer program was designed for determining the possible sequences of proteins from the partial sequences and mass values of their proteolytic peptides. Output data showed that the entire amino acid sequence of glucagon can be determined by only four and two cycles of degradation of chymotryptic and tryptic peptides, respectively.