Summary:Very high serum muramidase concentrations in the absence of renal impairment can serve as a marker of a number of haematological neoplastic disorders, and several cases are described where the presence of these disorders was indicated by this means before the diagnosis could be established on morphological grounds. The suitability of the muramidase assay for this purpose was supported by observations that muramidase concentrations in excess of approximately five times the mean normal value were correlated closely with either chronic granulocytic, acute monocytic or myelomonocytic leukaemia, myelofibrosis or polycythaemia vera. These neoplastic disorders appeared to be effectively distinguished by this criterion from other haematological and inflammatory disorders including reactive monocytic and granulocytic states which produced lesser elevation of the muramidase concentration.On the basis of the present experience, it is suggested that estimation of the serum muramidase concentration is particularly useful in the investigation of suspected leukaemia in refractory sideroblastic anaemia, persistent monocytosis, and pancytopenic states with cellular bone marrow where the morphological picture is difficult to interpret.