We have previously reported that in dissociated cultures of neonatal rat sciatic nerve, all of the cells could be identified by indirect immunofluorescence with two antisera to cell surface antigens. The Schwann cells, but not the fibroblasts, expressed the Ran-1 antigen, while the fibroblasts, but not the Schwann cells, expressed the Thy-1 antigen. We have exploited this difference to derive pure populations of Schwann cells. A combination of [3H]thymidine autoradiography and immunofluorescence marking showed that in Modified Eagle's Medium with 10% foetal calf serum, the Schwann cells divided slowly while the fibroblasts divided rapidly. Accordingly, two day old cultures were exposed to cytosine arabinoside to select against the fibroblasts, followed by growth in medium containing an extract of bovine pituitary which stimulated division of the Schwann cells. After 7 days the confluent cultures, which contained 80-90% Schwann cells, were passaged after treatment in suspension with antiserum to Thy-1 and rabbit complement. After continued growth in medium with pituitary extract, the secondary cultures contained greater than 99.5% Schwann cells. These purified populations have been maintained in culture for as long as 150 days (6 passages) and retained the Ran-1 marker. The cultured Schwann cells expressed the S100 antigen, as shown by indirect immunofluorescence and complement fixation, and receptors for cholera toxin. They did not express the large external transformation sensitive protein, the glial fibrillary acidic protein, or receptors for tetanus toxin.