Cultured Schwann cells were implanted into the thoracic spinal cords o f adult rats inside thin tubes made of polycarbonate film coated with poly-L-lysine. Additional control tubes were implanted which did not contain cultured Schwann cells. Some of the control tubes were coated with poly-L-lysine and others were not. One week to 2 months later the animals were perfused with fixatives and the tubes were prepared for light or electron microscopy. Immunocytochemical studies of the Schwann cell tubes reveal that they contain axons. Most of these axons are grouped in fascicles that run longitudinally through the tubes. The distribution of these axons matches precisely the distribution of basal lamina within the tubes as displayed by immunolabeling with an antibody to laminin. Surprisingly, the same patterns of labeling are seen in the control tubes, although they contain fewer axons. Control tubes lacking poly-L-lysine contain the fewest. Electron microscopy verifies that the tubes, including control tubes, contain Schwann cells and axons of different diameters. Furthermore, the Schwann cells ensheathe and myelinate the axons. These results strengthen the hypothesis that Schwann cells can support axonal growth in the spinal cords of adult animals. They also demonstrate that these Schwann cells can be implanted or they can be derived from the host animal. This finding raises the possibility that therapies could be devised for bridging spinal cord lesions that are based on maximizing migration of endogenous host cells into the sites of lesions.