Treatment of rabbit myelin basic protein component 1 with pepsin (enzyme:substrate, 1:500 w/w) in 0.5 M-ammonium formate (pH 6.00) for 15-20 min at room temperature resulted in limited cleavage of the protein. The resulting fragments were isolated by ion-exchange chromatography and gel filtration and identified by amino acid and COOH-terminal analyses and by tryptic peptide mapping. All of the possible products resulting from incomplete cleavages at the highly susceptible Phe44-Phe45, Phe87-Phe88, Leu109-Ser110, and Leu151-Phe152 bonds were isolated: peptides (1-151), (1-109), (1-87), (45-168), (45-151), (45-109), (88-168), (88-151), and (110-168). Of these, peptides (1-151), (1-87), and (88-151) were recovered in the greatest yield (0.14-0.19 mol per mol of starting protein). Relatively low yields (0.04 mol/mol starting protein) were obtained for peptides (1-109) and (110-168), indicating that the Leu109-Ser110 bond is somewhat more resistant to peptic cleavage than are the Phe-Phe and Leu-Phe bonds. Smaller fragments of the basic protein were also recovered: peptides (1-44), (1-28), (45-87), (88-109), (110-151), and (152-168). Many of the individual peptides could be readily identified in electrophoretograms of the total peptic digest. The relative electrophoretic mobilities of the above-mentioned peptides, together with the previously isolated peptides (1-14) and (15-44), were determined in 15% (w/w) polyacrylamide slab gels containing 1 M-acetic acid and 8 M-urea.