Oligopeptide mapping of nucleoproteins (NP proteins) of 54 strains of influenza A virus showed the presence of both common and individual oligopeptides. Using the distribution of variable oligopeptides as the criterion, NP proteins were subdivided into four groups (NP0, NP1, NP2 and NP3). The NP0 group is composed of H0N1 influenza viruses and the majority of animal influenza viruses. The NP1 group contains H1N1 (except A/California/78) and H2N2 influenza viruses isolated from man as well as H1N1 influenza viruses recently isolated from animals. A/New Jersey/76 (Hsw 1N1) influenza virus also belongs to the NP1 group. The NP2 group consists of H3N2 influenza viruses isolated from man and animals. A/California/78 (H1N1) influenza virus also falls into the NP2 group. The NP3 group contains NP proteins of four animal influenza viruses with antigenic formulas Hsw1N2, Hsw1Nav2, Hav6Nav5 and Heq2Neq2.