A method has been developed to quantitate the morphologically unrecognizable granulocytic/macrophage committed progenitor cell (CFUc). In man this is done by relating CFUc colonies cultured in vitro to marrow normoblast number determined by ferrokinetic measurements. In the mouse total hematopoietic precursors are determined by a radioiron dilution technique and CFUc are quantitated by calculating the percentage of colonies contained in the known number of nucleated cells plated. Like other hematopoietic precursors, CFUc numbers in 14 normal men and 10 normal mice were not significantly different, the mean being 0.85 and 1.4 x 10(7) CFUc/kg body weight, respectively. The validity of these values was substantiated by demonstrating an excellent correlation in the mouse between CFUc/kg body weight and CFUc/femur. In subsequent studies CFUc were quantitated in patients with lung cancer prior to and following aggressive combination chemotherapy when hematopoietic suppression was at a maximum. Compared to a mean value of 39 CFU/10(5) cells plated from chemotherapy the value 10 days later was significantly higher, the mean being 125/10(5) cells. That this reflected a relative change in the proportion of CFUc was suggested by the finding that 10 days after chemotherapy the mean value for CFUc/kg was 1.94 x 10(7) which was not significantly greater than a mean of 1.53 x 10(7) before treatment. These studies illustrate the advantages of expressing CFUc/kg body weight and indicate the usefulness of the approach in the evaluation of CFUc growth in a wide range of physiologic and pathologic conditions. In addition to better comparisons of results obtained in groups of individuals, multiple studies in single persons in whom hematopoiesis is not in a steady state will also be possible.