The Trembler PNS myelin-deficient mutant mouse offers a unique model for the study of axon-glial interactions. Previous work in our laboratory on Trembler mouse sciatic nerve established that myelinating Schwann cells exert a profound effect on the underlying neuronal cytoskeleton. Demyelinated axon segments exhibited decreases in the rate of slow axonal transport, axonal caliber, and neurofilament phosphorylation, as well as increases in neurofilament density. The present study considers effects on the microtubule cytoskeleton. At least two aspects of the microtubule cytoskeleton in Trembler PNS axons were altered by demyelination. First, the stability of the Trembler axonal microtubule cytoskeleton is decreased, as measured by decreased levels of insoluble tubulin (Sahenk and Brady, 1987). Second, the composition and phosphorylation of axonal microtubule-associated proteins, including tau, MAP 1A, and MAP 1B, are changed in Trembler demyelinated nerves. Further, the fraction of axonal tubulin moving at slow component b rates was increased (de Waegh and Brady 1990, 1991). These results provide further evidence that cell-cell interactions between myelinating glia and their underlying axons extend beyond a structural role, actively influencing biochemical and physiological properties of the axon.