The murine lympho-myeloid complex is depleted by a large dose of nitrogen mustard and the pattern of recovery of the haemopoietic tissues and cells followed to establish the relationships ox the precursor and mature cells. Culture of bone marrow in agar and stathmokinetic techniques are used to examine the controlled proliferation for the granulocyte elements in the bone marrow recovering from nitrogen mustard and the influence of the spleen is also considered. The control of the proliferation of the granulocyte cells is lost in myeloid leukaemia. Stathmokinetic methods and spleen colon assays are used to asses cell proliferation in irradiation-induced myeloid leukaemias in mice and the relevance of these parameters so measured to those demonstrated by normal bone marrow is discussed.