The life-span of platelets and of fibrinogen was studied by the use of 75Se-selenomethionine and 51Cr in 28 cases of acute leukaemia (AL), and in 2 cases of chronic myelocytic leukaemia with blast-cell transformation. The patients were grouped on the basis of clinical remission or activity. In all groups, whether examined by means of 75Se or 51Cr, a reduction of variable degree was found in platelet life-span, and platelet turnover was reduced in the majority of the cases. The sequestration data were atypical. Simultaneous 75Se studies for platelet and fibrinogen life-span revealed a latent diffuse intravascular coagulation (DIC) in three cases, the process being still in progress in one case and in remission in two cases at the time of observation. Fibrinogen life-span was reduced in some of the cases. Thrombocytopenia in AL is attributed to a complex origin in which infiltration of the bone marrow, the destructive effect of antimetabolic therapy, abnormally increased consumption of storage of the functionally impaired platelets, possibly latent DIC or episodic excessive sequestration may be equally involved. Remission may be accompanied by normalization of both thrombopoiesis and platelet kinetics.