We recently demonstrated that the serum amyloid A proteins (SAA) occur in six related polymorphic forms of indistinguishable molecular weights and COOH-terminal sequence. We have now obtained very homogeneous preparations of four of these proteins and shown that their amino acid compositions are similar but not identical. Two of these, SAA1 and SAA4, have the same 20 NH2-terminal residues despite striking differences in electrophoretic mobility and solution properties. SAA5 and SAA2, respectively, lack one and three of the NH2-terminal residues common to SAA1 and SAA4. The data are consistent with the postulate that some of the SAA polymorphs are products of different genes.