The numbers of peripheral blood (PB) granulocyte-macrophage colony forming units (CFU-GM) were evaluated in five patients treated with multiple weekly cycles of recombinant interleukin-2 (IL2). A 4.5-12 fold increase in the number of CFU-GM was evident within 8 days after the beginning of the treatment. The maximal increase in the absolute numbers of CFU-GM/ml blood caused by the IL2 treatment, ranged from 14 to 57 times the baseline values and was reached after two or three cycles of IL2. IL2-activated PBMC, added in vitro to the PBMC of a normal donor did not modify the number of CFU-GM present in the donor PBMC. CFU-GM were also recovered from frozen samples of in vivo IL2-activated PBMC.