IgG-positive astrocytes have been reported in scrapie-induced and Alzheimer's cortical plaques, multiple sclerosis, and CNS tissue around abcesses, metastatic tumors and primary tumors of glial origin. The present experiments ascertain if this immunoglobulin positivity is specific for these cases or a function of astrocytes around any site of injury in the CNS. The spinal cords of 30, 300-g Sprague-Dawley male rats were lesioned by passing a 26 gauge needle through the cord at T 6. After periods as long as 9 months, the spinal cords were processed for paraffin immunohistochemistry with antisera to IgM, IgG or double labeled for these immunoglobulins and GFAP (glial fibrillary acidic protein) a specific cytoplasmic marker for astrocytes. From 1 through 7 days after lesioning double labeled astrocytes in and around the site of injury are both IgM- and IgG-positive. From 14 days through 9 months postlesion, double labeled astrocytes surrounding the lesion are only positive for IgG. These data indicate a relationship between immunoglobulin availability, continued blood-brain barrier perturbation to immunoglobulins and the ability of reactive astrocytes to sequestor immunoglobulins. IgM is an early determinant for reactive astrocytes and IgG positivity is determinant for reactive astrocytes at any time period.