Dissociated neurons from chicken embryo dorsal root ganglia were cultivated in Rose chambers for up to 5 weeks. Newly formed fibers appeared as single fibers or grouped in bundles. During the first week of cultivation microtubules and microfilaments were frequently observed. The diameters of the fibers increased progressively and a number of varicosities appeared. In some distal portions of fibers large vesicles could be observed. During the second week in culture Schwann cells were easily recognized. They appeared as both dark and light cells. During this period myelinization of some fibers was seen to commence. High glucose concentrations were not observed to influence the process of myelinization. Essentially the important factors were the number of cells present in the culture and also the presence of NGF in the medium. Schwann cells myelinated nerve fibers exclusively. Two types of abnormalities in myelin formation were recognized: one Schwann cell myelinating two fibers and one fiber being myelinated twice, by two Schwann cells. Concomitantly with myelinization, myelin degeneration was observed. Histiotypic fascicles, typical constituents of the outgrowth zone of cultivated intact dorsal root ganglia, bundles of fibers, surrounded by connective tissue, are not formed. The surface ultrastructure of nerve fibers, as studied by scanning EM, was seen to be covered by numerous spherical elements as well as by small fibers and irregular elements. The growth cones of fibers were void of any glial contact. Myelinization occupied individual isolated segments along with nerve fibers and evidenced the absence of nodes of Ranvier. Relationships between single elements in the dissociated culture system are discussed, with respect to the possibilities for analysis of some of the elementary mechanisms of cellular and molecular interaction responsible for the development of the peripheral nervous system.