After treatment of acute leukaemia in adults with cytosine arabinoside and thioguanine complete and partial remission was achieved in 50% of cases. In view of short-lasting remissions achieved maintenance treatment with other cytostatic agents was necessary. The authors called attention to the fact that in the cases with remission the counts of granulocytes and platelets increased early during cytostatic treatment and the number of blast cells fell rapidly. In cases without response to this treatment these early signs of normalization of peripheral blood morphotic elements were not observed.