Thymosin beta 4, originally isolated from calf thymus [Low et al., Proc. Nat. Acad. Sci. USA 78, 1162-1166 (1981)] is present in a number of cell lines unrelated to the reticuloendothelium, including myoblasts and fibroblasts. It is also actively synthesized by these cell lines. Its content and rate of synthesis in the cell lines examined appear to be correlated with their ability to adhere and their motility.