The distribution of NP, P, and M proteins on Sendai virus nucleocapsids purified from cells and virions were studied by immunogold staining using monoclonal antibodies. NP molecules were found uniformly along the entire length of both cytosol and virion derived nucleocapsids. This observation is in accord with the earlier proposals that NP molecules maintained the structural integrity of the nucleocapsid. The distribution of P in nucleocapsids derived from the cytosol differed from the distribution in those originating from virions. In nucleocapsids derived from the cytosol, P molecules occurred in 4 to 10 discreet clusters at varying locations along the length of the nucleocapsid. In contrast, on nucleocapsids derived from virions, P molecules were uniformly distributed over the entire length of the nucleocapsid. These observations suggest that the distribution of P depends on the functional state of the nucleocapsid. The occurrence of P clusters at different locations on intracellular nucleocapsids indicates that P is a mobile molecule; this suggestion is consistent with P's role in viral RNA synthesis. The distribution of the matrix (M) protein also depended on where the nucleocapsids were derived from. Large quantities of M protein were found along the entire length of nucleocapsids derived from the cytosol, while in virion nucleocapsids, many fewer molecules of M were observed. The large amounts of M on the nucleocapsids originating from the cytosol supports the hypothesis that M protein mediates the recognition between the nucleocapsid and the envelope glycoproteins.