31 patients with successful kidney grafts were studied. Ep, Ht, Hb and T lymphocytes were determined. Native and grafted kidneys were studied by ultrasonography. After KT, 45% of patients had PTE and in 71% of these a spontaneous regression was observed. Mean serum Ep activity in patients with and without PTE was significantly higher than in healthy controls. Different erythroid colony growth sensibility and responsiveness to higher serum Ep (PTE and non-PTE patients) may be due to T3 cell interaction with BFU-E.