We have compared the immunolocalization of neurofilament protein (NF) with two other neuronal-specific intermediate filament proteins in large spinal axonal swellings (spheroids) of motor neuron disease and controls. All spheroids labeled each of the three different subunits of NF, the low, middle, and high molecular weight polypeptides. In doublelabel immunofluorescence, 300 axonal swellings were immunostained for NF, and 87% of them contained the intermediate protein peripherin. The pattern of immunostaining of NF and peripherin was indistinguishable at a high resolution viewed in 1 microns optical sections by confocal microscopy. A minority of the spheroids contained alpha-internexin, another intermediate protein, but it was weakly immunoreactive. The immunostaining of axonal swellings was similar for all epitopes tested in motor neuron disease and control subjects. The findings suggest that peripherin as well as neurofilament protein are major components of the proliferated intermediate filaments in spheroids.