The astrocyte cell lineage during postnatal development of the neopallium of Swiss mice was studied, using a colony culture assay method in which dissociated neopallial cells form discrete colonies in culture. It was found that immature epithelial-like cells that from type A colonies in culture come primarily from the subventricular zone but also from other regions of the neopallium. In culture, cells of type A colonies from type C colonies consisting of cells, which although still epithelial like, differ morphologically from the type A colony-forming cells. In the presence of dibutyryl cyclic adenosine monophosphate (dBcAMP) the type C colonies form cells rich in glial fibrillary acidic protein (GFAP) and stainable with Cajal's gold chloride sublimate, a stain specific for astrocytes. Therefore, it is proposed that type A colony-forming cells are astrocyte progenitor cells resembling the "pale" cells found in the subventricular zone (Blakemore and Jolly, '72), the "large glioblasts" (Sturrock, '76) and the free subependymal cells (Privat, '70; Paterson et at., '73) in the corpus callosum. The sequence of the lineage, i.e., cells forming type A colonies give rise to cells forming type C colonies which eventually differentiate into astrocytes, takes place in situ as well as in culture. As postnatal development of the neopallium progresses the number of colony-forming cells decreases in the subventricular zone and in other parts of the neopallium. The astrocyte progenitor cells migrate from the subventricular zone to other parts of the neopallium and progress through the lineage of differentiation in all regions of the neopallium.