To extend earlier observations on Schwann cell remyelination in multiple sclerosis (MS) lesions (Itoyama et al. 1983) we immunostained spinal cord sections from eight Japanese MS patients with antiserum to Po glycoprotein, a major constituent of peripheral nervous system (PNS) myelin, myelin basic protein (MBP), and glial fibrillary acidic protein (GFAP). Spinal cord sections from six of the eight Japanese MS patients contained large clusters of peripheral myelin sheaths with anti-Po immunoreactivity. In lesions found in four of the six patients, thousands of Po-stained PNS myelin sheaths were present. Necrosis was prominent in these lesions which included more than half of the spinal cord's transverse area. The number and density of regenerating myelin sheaths of peripheral origin were much greater than we observed in MS spinal cord lesions of white people (Itoyama et al. 1983). Anti-GFAP immunoreactivity was present in most brain and spinal cord lesions. However, the areas in lesions that contained large groups of PNS myelin sheaths lacked anti-GFAP immunoreactivity. Our data suggest that spinal MS lesions that are large, severely demyelinated, and partially necrotic may contain factors that inhibit fibrous astrogliosis. These factors, other substances in the large lesions and/or the lack of astrocytic scarring could then promote Schwann cell invasion, multiplication, and remyelination of surviving axons.