Labeling of platelets in vivo by 75Se-methionine was performed in premalignant and malignant haematological disorders for evaluation of the kinetics of platelet maturation. The "normal" platelet maturation time (time between the injection of the isotope and maximum radioactivity of separated blood platelets) in eight non-haematological patients showing normal platelet counts was 9.1 days. A shortening of platelet maturation time of 5-7 days was observed in three of four cases with panmyelopathy (high bone marrow cellularity), in three of four cases with malignant lymphatic disorders (multiple myeloma, chronic lymphocytic leukaemia, lymphosarcoma), and in two of four cases with myeloproliferative syndromes. No correlation to the peripheral platelet counts was observed. For explanation of the premature platelet release from the bone marrow a disturbance of the megakaryocyte maturation is suggested.