The production of basophils in semisolid agar cultures from normal [human] and chronic myeloid leukemia (CML) commited granulocyte-macrophage precursors was investigated using an original whole-dish staining technique with toluidine blue which produces a specific metachromasia in basophils. As additional proof of the basophilic nature of metachromatic cells, their degranulation after challenge with C5a anaphylotoxin and Synachten was observed.