AbstractA soluble, heat‐labile, trypson‐sensitive factor present in extracts of astrocyteenriched cultures enhances the expression of myelination‐associated events in rat brain cultures enriched in oligodendrocytes. Both the number of oligodendrocytes detected by immunofluorescence microscopy and the levels of biochemical activities associated with oligodendrocyte differentiation increased in the cultures in respose to the extract in a time‐ and concentration‐dependent fashion. The possible significance of these studies of normal oligodendrocytes function, and thus myelination, is