Granulocyte chalone is a cell line-specific, species non-specific regulator substance, which inhibits cell proliferation in the granulocyte system in a reversible manner. The existence and specificity of action of this substance has been shown both in vitro and in vivo in widely different assay conditions and using a broad spectrum of assay techniques. Granulocyte chalone inhibits cell proliferation in the transit populations of the granulocyte system; it seems to act at the level of cell membrane and divert cells from the G1 phase to the "G0" phase with some direct or indirect effect on S phase cells as well. The substance is non-cytotoxic and it has no detectable effect on any other tissue than normal and leukaemic granulocytes even after long-lasting in vivo treatment. Granulocyte chalone offers an exciting potential for the treatment of myeloid leukaemia and, therefore, the substance is of more than academic interest.