A combination of corticosteroids and vincristine achieved complete remission in 9 out of 15 cases of ‘agranular’ blastic crisis supervening in chronic granulocytic leukaemia, but failed to do so in other 9 cases in which the metamorphosis was of more progressive character and with a ‘granular’ cytomorphology. In the first group, although remissions were generally short, the completeness of their character made them fully worthwhile for the patients. The greater sensitivity of the undifferentiated, ‘agranular’ blast cell is not restricted to its reactivity to vincristine, but appears to be a general cytopharmacological feature