A growth cone-enriched fraction was prepared from 3-4 day rat cerebra. Examination of the growth cone fraction by electron microscopy revealed numerous structures circular in appearance that contain a number of features common to neuronal growth cones in vivo. The isolated growth cones stimulated a dose-dependent incorporation of [3H]-thymidine into cultured Schwann cells in a manner similar to that observed with an axolemma-enriched fraction prepared from adult rat brainstem. The mitogenic activities of both the growth cone fraction and axolemma-enriched fraction were decreased 50% and 20%, respectively, by treatment with heparitinase I. The mitogen for Schwann cells present in the isolated growth cones appears to be similar to that found in axolemma-enriched fractions prepared from adult rats.