The extension of cellular processes from the oligodendrocyte soma is an early and critical event in myelin formation. Previous reports from this laboratory have implicated a role for protein kinase C (PKC) as an important intracellular mediator of this critical step in myelinogenesis. In the current study, the regrowth of fibers by adult human oligodendrocytes was examined and was found to be significantly enhanced by the PKC stimulator, 4 beta-phorbol-12,13-didecanoate (PDB); this was accompanied by a 400-500% increase in oligodendroglial PKC activity. In contrast to other cell types, the increased PKC activity in oligodendrocytes was not followed by subsequent down-regulation of the enzyme. The role of PKC in oligodendroglial process formation was further demonstrated by the ability of inhibitors of PKC to block the basal- or PDB-enhanced fiber outgrowth. As well, studies employing isoform-specific agonists implicated PKC alpha as the major determinant of fiber outgrowth by oligodendrocytes. The potential significance of PKC in myelin formation was further underscored by the observation that the synthesis of myelin basic protein, a prerequisite component for myelinogenesis, was increased by 2-fold in PDB-treated oligodendrocytes. Collectively, these observations suggest that PKC, in particular the alpha isoform, constitutes an important mediator in the initiation of myelin formation.