Serum lysozyme (muramidase) estimation is a simple, convenient and useful laboratory investigation. A review of the literature shows that lysozyme has been implicated as an aetiological factor in various disorders, and credited with being a prognostic indicator in acute myeloid leukaemia, but these promises have not been fulfilled. This low molecular weight protein is found in the urine of some patients with renal tubular disorders, but some workers have emphasized its importance as a causal agent in hypokalaemia of acute myeloid leukaemia. Research should be concentrated on muramidase as an expression of cell functions rather than as an aetiological factor. Hypokalaemia in acute myeloid leukaemia may be caused by an unidentified substance of molecular weight similar to that of lysozyme.