Increased levels of soluble interleukin-2 (IL-2) receptor were found in plasma of six patients after the heart transplantation. The levels peaked the first week after transplantation and then gradually decreased to normal levels. The moderate episodes of heart transplant rejection did not affect the IL-2 receptor levels in comparison to transplant recipients without the rejection episodes. However, the transplant patients with pneumonia had the IL-2 receptor levels ten times higher than healthy controls. Hence, the IL-2 receptor levels in plasma might be a valuable indicator of the dynamics of the acute infection in heart transplant recipients.