The present invention is based on the discovery of polyclonal IgG's ability to promote Schwann cell maturation, differentiation, and myelin production. Methods for treating non-idiopathic, demyelinating peripheral neuropathies in mammals, where the neuropathy is not immune-mediated or infection-mediated, through the administration of polyclonal IgG are provided. Types of demyelinating peripheral neuropathies treatable with the present invention include peripheral nerve trauma and toxin-induced peripheral neuropathies. Alternatively, a composition of polyclonal IgGs can be applied directly to a peripheral nerve cell to induce maturation, differentiation into a myelinating state, and myelin expression or promote cell survival.