Platelet survival, platelet production and megakaryocyte quantifications were studied in control subjects and in patients with idiopathic thrombocytopenic purpura (ITP) and primary thrombocythaemia (PT). A platelet death probability function (DPF), which gives the expected probability of a randomly chosen platelet to be destroyed within the nextcoming short time interval, was constructed on the basis of the present knowledge of platelet function and the factors that influence its survival. Starting from this function platelet age and life span distributions and the function for the survival curve were derived. Platelet mean life span (MLS) was in controls 6.9d, in PT 5.2Dd and in ITP 0.44d. Determinations of platelet life span and platelet age distributions revealed a considerable variation in platelet life spans about these means and a great difference in platelet age compositions between the 3 groups of subjects studied. The function for the platelet survival curve, derived from the DPF, was employed in the calculation of platelet production and there was a highly significant relationship between platelet production and megakaryocyte volume per microliter bone marrow giving mutual confirmation to the 2 determinations and to the correctness of the death probability function.