The efficacy of restoration of cellular immunity in therapy of patients with certain chronic in-infectious or neoplastic diseases is under evaluation in several centers. Theoretically, a substance such as transfer factor is an ideal candidate for this therapy. Transfer factor, a dialyzable substance from blood leukocytes, selectively endows recipients with the cellular immune responses of the leukocyte donors, but does not induce antibody synthesis. Moreover, it does not contain intact leukocytes and therefore does not sensitize the recipient to HL-A antigens and cannot induce graft versus hostdisease. This review summarizes the current status of transfer factor therapy and points out the need for controlled clinical trials.