A micromethod was developed to grow granulocytic colonies from human bone marrow in agar-containing glass capillary tubes. Treatment of the bone marrow samples and the culture conditions (type and quantity of serum, CSF, cell seeding density) were optimized. Up to 60 colonies were obtained from 3.5 x 10(4) nucleated cells seeded into 50 microliter of total incubation medium/capillary with horse serum (13%) and leukocyte and partially purified bovine lung conditioned medium as CSF (17 and 3%, respectively). The micromethod requires less culture materials (about 1/20), cells, CSF and less time for colony counting, but higher cell densities for seeding, resulting in an increased sensitivity for drug or factor testing. Colony morphology can be easily examined. The micromethod offers further advantages, e.g. quantitation by light scattering densitometry, and hence seems suitable for clinical investigations.