The composition of the large helical internal components of influenza virus was investigated by immunogold labeling/electron microscopy with antibodies to the nucleoprotein (NP), matrix protein (M), and polymerase complex (PB1, PB2, and PA) of the virus. The morphologically intact helices, obtained by air-drying of the virions on the electron microscope grid, showed little or no labeling with any of the above antibodies. However, partial to full degradation of the helix by proteinase K (2 ng/ml) prior to immunogold labeling made the helices accessible to all three antibodies. The results are consistent with a model that the helix represents a polymer of M protein enclosing or containing the influenza ribonucleoprotein(s).