Toward understanding the early molecular development of neocortical neurons, we report the generation of two clonal murine cell lines derived by sequential oncogenic retroviral infection of neocortical neuronal precursors. The resulting cell lines stably express the telencephalon-specific gene BF-1 and a gene enriched in the neocortical ventricular zone, vzg-1. They also express early neuronal but not glial markers, possess neuron-like processes without chemical synapses by ultrastructure, and appear pyramidal or bipolar with processes resembling apical dendrites, bifurcating and beaded axons, or growth cones. Immortalization appears to have occurred through the expression of oncogene combinations SV40 large T with vras or SV40 large T with both vsrc and vmyc. These cell lines may represent developing neocortical neuroblasts blocked from complete differentiation, and they should be useful in the isolation and analysis of genes involved in early neocortical development.