The internal matrix proteins of A/Okuda/57, A/Finland/4/74 and A/New Jersey/8/76 viruses and several recombinant strains have been examined by radioiodination of the purified proteins followed by 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 the origin of the matrix proteins of the recombinant viruses to be determined. The use of matrix protein identity as a marker in recombination work and its possible correlation with biological properties of the virus is discussed.