The effects of providing low oxygen tension in the gas phase of two different types of cell culture systems were investigated. The clonal growth of granulocyte‐macrophage progenitor cells in an agar culture system was improved markedly by incubation within a low oxygen tension gas phase (48 mmHg – 6.8%) instead of the conventional air (135 mmHg – 19%), the effects being measured by increases in numbers of colony forming cells detected and in the colony sizes. The increased efficiency of colony formation was observed both with mouse and human marrow cells. A similar effect was observed in a liquid adherence culture system with primary cultures of foetal mouse fibroblasts both at clonal and higher cell densities.