Remyelination of central nervous system axons was promoted by systemic injection of serum immunoglobulin G (IgG) from donor mice hyperimmunized with homogenized spinal cord. In SJL/J mice infected with Theiler's virus, primary demyelination and inflammation in the spinal cord were extensive. Following treatment with IgG to spinal cord homogenate, new myelin synthesis as measured by quantitative morphometry increased sixfold. Cells in areas of remyelination that incorporated [3H]thymidine did not express differentiation markers of macrophages, oligodendrocytes, or astrocytes but were identified by electron microscopic autoradiography as progenitor glial cells. These findings raise the possibility that IgG secreted in demyelinating lesions might have the potential to promote myelin repair.