An in-vitro culture system is described in which metachromatically granulated leucocytes differentiate from human bone marrow under stimulation by conditioned medium from the 5637 cell line. By morphological, cytochemical and immunophenotypic criteria these were shown to be basophils, rather than mast cells. This system may prove useful in studies of basophil differentiation, and in characterisation of growth factors active upon basophil precursors. The study also identified the 5637 bladder carcinoma cell line as a constitutive producer of basophil growth factor(s); this may represent a convenient source for the isolation and purification of such factors.