An unusual polymorphic protein family of nine or more variants has been isolated from the byssal adhesive plaques and foot of the marine mussel Mytilus edulis. In accordance with established terminology, the family is referred to as M. edulis foot protein 3 or simply Mefp-3. Variants of Mefp-3 have molecular masses of about 6 kDa, isoelectric points greater than 10.5, and an amino acid composition dominated by six amino acids: glycine, asparagine, 3,4-dihydroxyphenylalanine (Dopa), tryptophan, arginine, and an unknown basic amino acid. The latter has been isolated and identified as 4-hydroxyarginine using fast atom bombardment mass spectrometry and appropriate standards. The primary structure of variant Mefp-3F has been determined by peptide mapping using automated Edman sequencing in combination with fast atom bombardment and matrix-assisted laser desorption ionization mass spectrometry: ADYYGPNYGPPRRYGGGNYNRYNRYGRRYGGYKGWNNGWNRGRRGKYW where Y represents Dopa, and R represents hydroxyarginine. Notably, the 4 occurrences of RY are marked by a resistance to trypsin digestion. Although the conversion of tyrosines to Dopa is essentially complete, hydroxylation of arginines varies between 40 and 80%. In contrast to other mussel adhesive proteins such as Mefp-1 and -2 which have large numbers of highly conserved, tandemly repeated peptide motifs, Mefp-3 has only short sporadic repeats. The specific function of Mefp-3 in byssal adhesion is unknown.