The polymerase proteins (PB1, PB2, PA) of the influenza virus strain A/PR/8/34 (H1N1) were isolated from whole virion or ribonucleoprotein (RNP) fractions by electrophoresis on polyacrylamide gel and electroelution or Sepharose CL-6B chromatography in the presence of SDS. Antisera to polymerase proteins (P proteins) were raised in rabbits; the immunoglobulins (Ig) were purified by affinity chromatography. Characterization of the antibody fraction by Western blot analysis showed a highly monospecific reaction with the three polymerase proteins. Spot immunobinding assay was used to compare the immunoreactivity of the monospecific polymerase antibodies with the P proteins of other influenza A subtypes and influenza B strains, revealing high immunoreactivity with the components of all influenza A strains and only insignificant reactivity with the components of the influenza B strains tested.