GM-CSF was used to overcome fatal myelosuppression after cytotoxic chemotherapy. Two different application modalities were compared: a continuous 24 h infusion was more effective compared to a 30 min short term infusion. Using the former modality at a dose of 10 μg/kg/d for five days very impressive responses were observed. No major side effects did occur. The first 13 patients treated in this way included 8 AML cases. Only one of these latter patients had a leukemia relapse. However, in this patient the immediate GM-CSF response was clearly separated from the relapse occurring later in the observation period. Thus, the preliminary results of the present paper suggest that GM-CSF besides of being very efficient in accelerating granulocyte recovery does not stimulate the growth of AML blasts in vivo in patients with only minimal residual disease