Human fibroblast colony formation from bone marrow was performed in liquid culture. Fetal calf serum was used as a stimulator of the fibroblast colony formation. The colony formation took place not only in normal donors, but also in patients with acute leukemia and chronic myelocytic leukemia. At the diagnosis of the disease, significant colony suppression was observed in most cases of acute leukemia, while the number of colonies increased in half of the cases of chronic myelocytic leukemia. However, there was no correlation between the colony-forming efficiency and the initial number of peripheral platelets or bone marrow megakaryocytes that contained growth-promoting factor. The number of colonies increased after chemotherapy, recovered at the stage of complete remission, and then decreased to low levels at relapse in the patients with acute leukemia; it decreased after treatment with busulfan in the patients with chronic myelocytic leukemia. This fibroblast culture method is useful for counting fibroblast colony-forming cells in the bone marrow of human leukemia.