This report describes a patient with small-cell carcinoma of the lung associated with blindness. The serum of this patient was tested for immunoreactivity with retinal sections because there was no evidence of tumor metastasis to the central nervous system and because neuroendocrine cells of the lung share common antigens with retinal neurons. The immunoglobulins in the sera from this patient, from two other patients with small-cell carcinoma, from two patients with multiple sclerosis and from three controls were reacted with 8 micrometers thick sections of retina. Dog, cat and human retinal sections were used. Antihuman immunoglobulins that were conjugated to horseradish peroxidase were used as the second antibody to identify the cells which bound the patients' immunoglobulins. A high titer (1:500) antibody level against retinal ganglion cells was found in the patient with small-cell carcinoma and blindness; the antibody reaction was similar with retina from the three species. The large ganglion cells bound the immunoglobulins of the patients with small-cell carcinoma and blindness while the immunoglobulins from the control and multiple sclerosis subjects did not bind these cells selectively at high dilutions. It remains to be shown whether these antibodies have an etiologic significance in the development of blindness.