Mass spectrometry was used to verify portions of a proposed amino acid sequence of the bacteriophage P22 tail protein which had been inferred from the DNA base sequence. The exopeptidases dipeptidyl aminopeptidase I and IV and dipeptidyl carboxypeptidase were used to hydrolyse intact protein and fragments generated by cyanogen bromide treatment of the tail protein. After partial purification by high performance liquid chromatography, peptides were identified by gas chromatography/mass spectrometry and fast atom bombardment mass spectrometry. The results indicate that the initiation amino acid, N-formylmethionine, has been removed from the N-terminal of the protein and that the protein ends at the termination codon which is 667 amino acids from the N-terminal residue. Nine regions of the protein from 13 to 42 residues in length were verified. All of the sequences checked were in the same DNA reading frame and corresponded to the proposed sequence.