The M1 matrix protein of the influenza virus is one of the main structural components of the virion that performs several different functions in the infected cell. X-ray analysis (with 2.08 Å resolution) has been performed for the N-terminal part of the M1 protein (residues 2-158) but not for its C-terminal domain (159-252). In the present study, we analyzed the structure of the M1 protein of the influenza virus A/Puerto Rico/8/34 (H1N1) strain in acidic solution using tritium planigraphy. The incorporation of tritium label into the domains of the M1 protein were studied; the C domain and the interdomain loops are preferentially accessible to tritium. Analytical centrifugation and dynamic laser light scattering demonstrated anomalous hydrodynamic parameters and low structuredness of the M1 protein, which has also been confirmed by circular dichroism data. Bioinformatic analysis of the M1 protein sequence revealed intrinsically unstructured segments that were concentrated in the C domain and interdomain loops between the N-, M-, and C domains. We suggest that the multifunctionality of the M1 protein in a cell is determined by the plasticity of its tertiary structure, which is caused by the presence of intrinsically unstructured segments.