An extracellular matrix protein, cytotactin, with widespread tissue distribution has been identified, isolated, and partially characterized. Cytotactin mediates glia-neuron adhesion in vitro, but unlike Ng-CAM, the neuron-glia cell-adhesion molecule, it is absent from neurons. Cytotactin was isolated from 14-day embryonic chicken brains as structurally related polypeptides of Mr 220,000, 200,000, and 190,000. These polypeptides were efficiently extracted in the absence of detergent and appeared to be disulfide-linked into higher polymers. Immunofluorescence staining with specific antibodies indicated that cytotactin is found in extracellular spaces and in basement membranes of a variety of non-neural tissues including smooth muscle, lung, and kidney. In the cerebellum, it appears on glial end-feet, on Bergmann glial fibers, and in extracellular spaces. The molecule is synthesized by glia and cells from smooth muscle, lung, and kidney. It is found at the surface of glia in culture in a cell-associated fibrillar pattern. A survey of the times and sites of its appearance during embryogenesis is consistent with the hypothesis that cytotactin is a cell-substrate adhesion molecule that may mediate cell migration in a site-restricted fashion.