Leucocyte alkaline phosphatase (LAP) was histochemically detected in 7 to 18 % of cells in tissue culture lines derived from the peripheral blood or bone marrow of each of 5 patients with untreated acute myelogenous or monomyelogenous leukaemia and in 30 % of cells in a clonal line of a rat promyelocytic leukaemia. Following transfer to diffusion chambers intraperitoneally implanted into total body irradiated rats, LAP levels were detected in up to 92 % of human and 80 % of rat leucocytes. There was no associated morphologic differentiation. In rat leukaemia cells peroxidase and myeloid specific esterase also increased from tissue culture levels. Return of cells to tissue culture decreased enzymes to pre‐implant levels. Addition of plasma or peritoneal fluid from irradiated rats to cells in tissue culture again induced LAP. In contrast, LAP was not increased under these conditions with cell lines derived from patients with acute lymphatic leukaemia, or Sezary cell leukaemia. These studies indicate that a humoral factor in peritoneal fluid and plasma of irradiated rats increases LAP in human as well as rat leucocytes.