This review summarizes available literature regarding immunoregulatory activities of calf thymus factor X (TFX®) in vitro and in vivo, both in animal and human models, as well as its therapeutic efficacy evaluated in a number of clinical trials. TFX® is a safe, non toxic preparation of thymic hormones from juvenile calfs’ thymic glands, capable of restoring immune system functions in immunocompromised animals and patients by acting not only on immature T cells but also indirectly on maturation and function of other cell types. A potential, preventive or therapeutic utility of TFX® could find most optimal application in combating immunodeficiency, autoimmune disorders and restoration of full immune competence following chemo- or radiotherapy. More research is needed to explain its molecular and cellular mode of action after a long period of inactivity in this field.