The retinoic acid-responsive thymosin beta-10 gene is known to be developmentally regulated in the human brain. We now report the novel finding that thymosin beta-4, a structurally related 5-kDa actin-sequestering protein, is also subject to a similar but not identical pattern of expression during normal human neuroembryogenesis. However, while thymosin beta-10 mRNA was undetectable (by northern blot analysis) in adult human brain, levels of thymosin beta-4 mRNA, although greatly reduced, were still present. Moreover, a novel thymosin beta-10-like gene was also found to exhibit a unique stage-specific expression during early human neural development. These experiments, together with previous findings, indicate that the products of the two thymosin genes, possibly in association with cytoskeletal elements, may play different roles during early neuroembryogenesis and neural maturation.