Compared with the common agar of hematopoietic colonies in Petri dishes, the use of glass capillary tubes offers a couple of advantages which have been exploited. Among these advantages are less materials, less cells to be seeded, lower costs, easier and quicker colony counting by visual and semi-automated optical scanning, and lower risk of bacterial contamination. In tubes several parameters affecting the colony formation of mouse and human granulocytes and macrophages as well as of human peripheral T and B lymphocyte from mouse spleen were optimized. The applicability of the cultures to assay for inhibiting and stimulating humoral factors and cytostatics was demonstrated.