Kinetics of circulating haematopoietic progenitors was analysed during chemotherapy- or chemotherapy plus granulocyte colony-stimulating factor (G-CSF)-induced mobilization of peripheral blood stem cells. Circulating progenitors including colony-forming unit granulocyte/macrophage (CFU-GM), burst forming-unit erythroid (BFU-E) and multilineage colony forming unit (CFU-Mix) were studied serially on alternate days during a recovery phase from chemotherapy for consolidation of complete remission. In 18 patients with acute leukaemia, 27 courses of consolidation chemotherapy were performed with a combination of an intermediate-dose cytosine arabinoside with etoposide (Ara-C/Etop) or mitoxantron (Ara-C/Mit). G-CSF (5 micrograms/kg) was administered during the recovery phase in 6/14 courses with Ara-C/Etop and in 4/13 courses with Ara-C/Mit. G-CSF induced a significant and synchronized increase of circulating CFU-GM, BFU-E and CFU-Mix by more than 4-fold at their peaks. The peak of CFU-GM was significantly correlated with that of both BFU-E and CFU-Mix, irrespective of additional G-CSF mobilization. G-CSF also produced a significant increase of monocytes in a synchronized fashion with an increase of circulating CFU-GM. Interestingly, peripheral blood monocytes spontaneously produced high concentrations of IL-6; a significant correlation was observed between absolute monocyte counts and plasma levels of IL-6 or peak levels of CFU-GM. These observations indicate that the addition of G-CSF to chemotherapy-induced mobilization can facilitate further expansion of a blood progenitor pool during the haematopoietic recovery, probably through the stimulation of monocytes to proliferate and to induce their monokine production such as IL-6. The data also suggest that absolute monocyte counts may be a useful indicator to predict the peak of circulating progenitors for collecting autologous blood stem cells.