A technique for growing human bone marrow cell colonies in agar-gel medium is described. “Feeder layers” containing 1 × 106 normal human peripheral white blood cells are used as the stimulus for colony growth. Human bone marrow aspirates are collected in heparinized syringes and plated as 2 × 105 cells on “feeder layers.” Normal human bone marrow yields 32–102 colonies per 2 × 105 cells plated. Colonies are almost exclusively granulocytic. Growth rate of colonies is slower than with mouse bone marrow but colonies reach a comparable size (500–1500 cells) at days 12–16.