The analysis of single haemopoietic colonies grown in methylcellulose and in agar was performed at intervals by a cytocentrifugation method. Correlation was established between morphology of the whole colonies and their cellular content. Three main cell lines predominated: neutrophils, macrophages, eosinophils; a few colonies contained a pure population of basophil-like granulocytes. Development was followed from myeloblasts to polymorphs, both being present in most of the colonies. Cumulated results showed that (1) the proliferating compartment remained quite large till day 14, with 58% of cells in S phase, and rapidly decreased after day 16, and that (2) the polymorphs rapidly disappeared from the culture medium. Differentiation proceeded at different rates from one colony to the other, thus suggesting heterogeneity between colony forming cells (CFC). Neutrophil colonies appeared and lysed more rapidly than did eosinophil colonies. Macrophages arose from large immature cells with many promyelocyte features; such cells were present in mixed colonies, containing both neutrophils and macrophages. It is very likely that granulopoiesis results from the development of distinct committed CFC. This work was carried out using normal human bone marrow and may be a useful tool for studying pathological material in the future.