Porcine C3a was generated in whole porcine serum by inulin activation of enzymes of the alternative complement pathway. The C3a anaphylatoxin was isolated according to the procedures previously described by Hugli. The complete amino acid sequence for porcine C3a was determined utilizing automatic sequencing techniques in addition to manual subtractive Edman degradation and carboxypeptidase A, B, or Y digestion of isolated peptides. Porcine C3a is composed of a polypeptide chain containing 77 amino acid residues and has a m.w. of approximately 9,000 daltons. This C3a molecule is devoid of threonine, tryptophan, and carbohydrates. The proposed primary structure for porcine C3a is as follows: (see article) Comparisons between the amino acid sequences of human and porcine C3a reveal that the six half-cystinyl and five aromatic residue positions are conserved. Conservation of these six half-cystinyl residue positions suggest that the disulfide arrangement remains identical in both anaphylatoxin molecules. Maintenance of three interconnected disulfide linkages helps to explain a near identity between the secondary structures of human and porcine C3a as indicated by circular dichroism measurements. Particular attention was focused on the COOH-terminal region of the anaphylatoxins since an arginyl residue at position 77 is functionally essential in both human and porcine C3a. Five residue positions at the carboxy termini were conserved in both C3a molecules, and the sequence Leu-Gly-Leu-Ala-Arg probably relates directly to anaphylatoxin activity. A total of 23 residue replacements occur between human and porcine C3a which accounts for a 30% difference in primary structure. Although the C3a molecules exhibit identical biologic activity, this rather large structural difference readily explains the absence of a detectable immunologic cross-reactivity.