The effects of chemotherapy on the growth of colony forming unit-granulocyte macrophage (CFU-GM) from the bone marrow (BM) and peripheral blood (PB) of 8 patients with lung cancer were studied by using recombinant interleukin-3, granulocyte macrophage-colony stimulating factor, and granulocyte-colony stimulating factor. After chemotherapy, the kinetics of CFU-GM depend on the type of chemotherapeutic regimens used: a rebound overshoot of CFU-GM in the PB was seen in the 4 patients treated with cisplatin plus etoposide (PVP) but not in the 4 patients treated with cisplatin plus mitomycin plus vindesine (CMV). In the BM, the recovery of CFU-GM was more retarded in patients treated with CMV relative to that seen in patients treated with PVP. The results obtained in simultaneous cultures revealed the differences between CMV and PVP in the kinetics of chemotherapy-induced myelosuppression. Determination of the optimal timing of in vivo use of hematopoietic growth factors after cytoreductive chemotherapy requires careful experimental design.