Monospecific antisera for the influenza polymerase proteins and high resolution immunoelectron microscopy have been used to investigate the topographical distribution of the polymerase molecules on influenza ribonucleoproteins (RNPs). Antibodies to PB1, PB2, and PA identify a single polymerase binding site located at, or very close to, the end of each RNP. Double labeling experiments confirm that all three polymerases are at the same end of each RNP and that they are in close association.