Methods are described for the preparation in vivo of 35S-methionine-labelled influenza viruses, the purifiction of the nucleoprotein (NP) and matrix (M) proteins and the separation of peptides obtained by protease digestion by two-dimensional thin-layer chromatography. The maps of the M proteins of A/Okuda/57(H2N2) and A/Finland/4/74(H3N2) were very similar overall but differed in three peptides. Hence they could be clearly distinguished. Maps of the NP proteins of the same strains showed a greater number of differences. A recombinant strain having the haemagglutinin and neuraminidase of the A/Finland/4/74 parent and the virulence of the A/Okuda/57 parent was shown to have the M and P proteins of A/okuda/57.