Lymphocyte subpopulations were determined in 13 patients, recipients of kidney allografts, 7 of them during an acute rejection episode (ARE). Monitoring of the T lymphocyte suppressor or T helper cells was performed by aid of the theophylline sensitivity test and the local xenogeneic graft-versus-host reaction (GVHR). An absence or a striking decrease of theophylline-sensitive T suppressor cells was found in all patients during ARE. Incubation of the lymphocytes of these patients with a thymic hormone, THF, raised the number of TS lymphocytes from nil or from a very low level to normal or above. The therapeutic use of THF in selected renal allograft recipients is proposed.