A microassay for human committed progenitor cells (CFU-c) has been developed using 24-well, 16 mm diameter culture dishes. Comparisons were made of simultaneous cultures of 21 samples in both 35 mm and 16 mm culture dishes employing two sources of colony-stimulating factor (CSF). The microassay does not differ significantly from the standard 1 ml 35 mm assay, apart from some enhancement of colony numbers in the 16 mm dish. Other advantages of the microassay are that it is economical with respect to cells, media, and space; and it is possible to increase the number of experiments fivefold which can be performed with the same number of cells.