Patients with diseases associated with defects in cellular immunity, such as the Wiskott-Aldrich syndrome, characteristically have severe recurrent infections and usually succumb to overwhelming infection at an early age. This communication describes a patient with this syndrome, defective delayed hypersensitivity by skin tests and by in vitro lymphocyte response, who was treated with dialysate of peripheral blood leukocytes (transfer factor). After treatment, the clinical status of the patient improved dramatically, concomitant with the development of delayed hypersensitivity to antigens to which the donor was sensitive. In vitro tests after transfer indicated that the patient's lymphocytes, when stimulated by specific antigen, produced migration inhibitory factor without concomitant DNA synthesis. These observations dissociate skin test sensitivity and activity of migration inhibitory factor from in vitro blastogenesis. Further, the response to phytohemagglutinin remained diminished before and after therapy. While these findings represent only an individual case, the climical results suggest that investigation of the use of transfer factor appears warranted in the therapy of Wiskott-Aldrich syndrome and other genetically-determined diseases associated with impaired cellular immunity.