The frequency of large granular lymphocytes and their relationship to functional NK-activity as assessed by the capacity to lyse the K562 tumour target was analysed in five allogenic and two autologous human bone marrow transplant recipients. Date revealed: almost identical disappearance and reconstitution of both parameters further indicating that LGL represent effector cells of spontaneous lysis of K562 targets; a long-lasting suppression of the absolute numbers per ml of blood of both LGL and functional NK activity which we believe was not the consequence of reconstitution with immature effector cells but rather reflected immunosuppressive therapy; LGL exhibits the fastest reappearance rate subsequent to total body irradiation of all populations of circulating leucocytes.