Transfer factor was first discovered by Lawrence in 1955, but was not used therapeutically until 1969 when we reported its use in a Wiskott-Aldrich patient. Since that time, it has been used in a wide variety of disorders related to defects in cellular immunity, infectious diseases, and malignant diseases. This report describes our experience with transfer factor. Report number I discusses rationale for patient selection, procedures for transfer factor therapy, procedures for monitoring the efficacy of therapy, untoward effects of therapy, and experience with transfer factor therapy in severe combined dual system deficiency disorder. The results of our study on transfer factor therapy indicate that it is capable of inducing a clinically acceptable level of cell-mediated immunity in approximately 50% of patients with a variety of immunodeficiency disorders. It also appears to be a useful adjunct to chemotherapy, and may possibly act synergistically with transplanted fetal thymocytes to produce a constantly regenerating specifically competent source of T lymphocytes, thereby obviating the need for bone marrow transplant for severe combined dual system deficiency disorder.