In the peripheral blood of healthy adults the number of theophylline-resistant T-lymphocytes (T-res) is 51 +/- 4% and 1,161 +/- 326/microliter, respectively, and the number of theophyllins-sensitive 11 +/- 2% and 252 +/- 95/microliters. The membrane markers show a heterogeneous distribution. The quotient from T-res/T-sens is 4.9 +/- 1.3. Within the T-sens 14% are CD4+ and 30% CD8+, 29% carry Fc-IgC and 34% Fc-IgM-receptors. Thus the T-res in their netto-function were helper cells and the T-sens suppressor/cytotox cells. Before the transplantation the preoperative ratio of the two subpopulation is significantly diminished (Q = 3.66 +/- 1.53), however, prognostic statements cannot be deduced from this. Activations of the immune system (rejection crises, cytomegalovirus infections) are accompanied by significant diminutions of the T-sens, whereby the T-res/T-sens-Quotient increases. Thus they are unequivocally included into immunoregulatory processes.