We have examined the matrix proteins of A/Okuda/57, A/Finland/4/74 and A/New Jersey/8/76 viruses and several recombinant strains by radioiodination of the purified polypeptides followed by tryptic peptide mapping. The method is rapid and requires only small amounts of material. Reproducible differences were detected between the matrix proteins of the above parents and allowed origin of the matrix proteins of the recombinant viruses to be determined. The possible use of matrix protein identity as a marker in recombination work is discussed.