Mouse bone marrow cells were grown in semi-solid agar contained in glass capillary tubes. Several parameters affecting colony formation in the capillaries were studied. 104 cells in 100μ1 incubation medium within one capillary produced 22 to 30 colonies of granulocytes and macrophages. Compared with the common petri dishes glass capillaries offer several advantages under the conditions used: 1. A twofold higher plating efficiency. 2. Applicability to optical scanning by light scattering and electronic counting, allowing automation and greatly improving sensitivity, statistical accuracy and reproducibility. Kinetics of colony growth can also be monitored. 3. Diminished risk of bacterial and fungal contamination. 4. A more than tenfold lower need for materials on similar statistical errors. Substituting methylcellulose for agar resulted in colonies of fibroblast-like cells adherent to glass surface. Glass non-adherent cells showed a threefold higher plating efficiency in agar.