Trembler mouse, a Schwann cell mutation, is characterized by severe hypomyelination of peripheral nerves, high Schwann cell proliferation and the presence of a multilayered basal lamina which surrounds them. In contrast with their continuous in vivo division, mutant Schwann cells prepared from 15-day sciatic nerves display a lower proliferation rate in cell culture than normal Schwann cells. However, quiescent Trembler Schwann cells are still able to respond, as normal Schwann cells, to exogenous mitogens, such as nerve extracts and myelin-enriched fractions. In addition, both normal and Trembler Schwann cells proliferate in response to Trembler serum. Fibroblast growth factor is not the mitogenic factor which stimulates mutant Schwann cell proliferation in vivo, since it is absent in Trembler serum and poorly concentrated in Trembler adult sciatic nerves. Our results suggest that, in vivo, the serum of Trembler mouse probably contains mitogenic factors, not yet characterized, which may trigger the permanent division of mutant Schwann cells, in contrast to the quiescent state of these cells in the nerves of normal mice.