The levels of circulating hematopoietic progenitors were measured in 28 patients with primary lung cancer. The average numbers of progenitors per milliliter of blood were 33 (range 0-360) for colony-forming unit-granulocyte macrophage (CFU-GM), 23 (range 0-140) for burst-forming unit-erythrocyte (BFU-E), and 4 (range 0-50) for colony-forming unit-mixed lineages (CFU-mix). No significant influence of age, sex, histological type, or clinical stage of the tumor on the progenitor levels was detected. After cytoreductive chemotherapy of the patients by treatment with cisplatin plus etoposide, the cells showed 6- to 50-fold rebound overshoots, but no rebound was observed after treatment with cisplatin alone, cisplatin plus mitomycin C or cisplatin plus vindesine plus mitomycin C, or in 4 of 5 patients treated with cyclophosphamide plus adriamycin plus vincristine. Peripheral blood hematopoietic progenitors should be useful as an alternative source of stem cells for lung cancer patients treated with marrow ablative chemotherapy.