Circulating numbers of committed granulocyte-monocyte hematopoietic stem cells (CFUc) were measured in the peripheral blood of 20 patients with extensive-stage small cell lung carcinoma during induction chemotherapy. All patients received cyclophosphamide, doxorubicin, VP16–213, and vincristine. CFUc measurements were made either weekly or twice weekly. As leukocytes declined following chemotherapy, circulating CFUc numbers also declined. However, as leukocytes recovered from their nadir levels, circulating CFUc numbers per mononuclear cell and per ml of whole blood became substantially expanded in 19 and 17, respectively, of the 20 patients studied. Per mononuclear cell, the median CFUc expansion was 7.9-fold, and the highest expansion seen was 157-fold. Per ml of blood, the median CFUc expansion was 6.7-fold, and the highest expansion seen was 46-fold. The magnitude of the amplification, its occurrence in 85 to 95% of patients studied, and its association with leukocyte recovery strongly suggest that appropriately timed collections of peripheral blood mononuclear cells obtained during leukocyte recovery from nonablative chemotherapy could be used to provide hematopoietic stem cells in numbers sufficient to effect hematopoietic reconstitution after subsequent marrow-ablative therapy.