We found an antinuclear antibody highly restricted to nuclei of neurons in two patients with subacute sensory neuronopathy complicating oat cell carcinoma of the lung. Serum was tested by indirect immunofluorescence and immunoperoxidase staining. At low concentrations of antibody, only the nuclei of the neurons were stained. At high concentrations, there was also staining of the nuclei of glial cells and fetal nonneural tissues. The cytoplasm of most neurons was stained with the immunoperoxidase method.