Clusters of cells obtained from a short-term culture of human bone marrow cells in methylcellulose were transplanted in cell-free plates containing conditioned medium. Of 2559 transplants, 439 gave small clusters, 223 gave large clusters, and 70 gave colonies. A better cloning efficiency was achieved from 4-day-old clusters, from aggregates containing 3 cells or more, and from large cells rather than from small cells. Cytocentrifugation could be performed on 215 large clusters or colonies. The population consisted of pure neutrophils in 36.3% and of pure macrophages in 36.8%. A mixture of neutrophils and macrophages was found in 23.2%, thus indicating that these two cell lines originated from the same committed cell (G-CFC). Eosinophils were found in 8 (3.7%) clusters as a pure population, and were never mixed with either neutrophils or macrophages in the other 207 clusters. Despite the small number of eosinophil colonies, one could suggest that using the in vitro technique the committed eosinophil colony-forming cells could be distinguished from the G-CFC.

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