Immunomorphological analysis revealed the presence of thymalin in human epidermis and in fetal reticuloepithelium. These structures are developed from the common embryonic primordium ectoderm. In embryos and adult humans thymalin is present only in young epidermal cells, which undergo age-related involution. By the age of 70 years, the layer of thymalin-containing cells looks thinned and discontinuous. The content of thymalin, a thymic factor, decreases with age.