Total serum alkaline phosphatase activity and its isoenzymes were studied in 126 patients sent to hospital with a neoplastic blood disease. They comprised 63 patients with a lymphoreticular neoplastic disease, 17 with immature stem-cell leukaemia, 11 with a plasmocytoma, 16 with lymphocytic leukaemia, 4 with polycythaemia and 9 with the myelofibrosis syndrome. Evaluation of the serum alkaline phosphatase isoenzymes significantly enhances the efficiency of the diagnosis and the organ specificity of the enzymes in these conditions. It is particularly important in the above states for studying liver and bone involvement, both from the aspect of the organic localization of the disease and from the aspect of evaluation of the success of therapy.