Muramidase (lysozyme) is a hydrolytic enzyme present in monocytes and all members of the granulocytic series from pro-rnyelocytes to neutrophil polymorphs. Measurable quantities of this enzyme are also present in serum, and in normal humans the level of serum muramidase correlates with the absolute granulocyte count. Studies performed by other investigators have related serum muramidase to leukaemia, and experimental work, in particular that performed by Fink and Finch, has demonstrated a correlation between serum muramidase and granulocyte turnover. In the Clinical Haematology Unit at Sydney Hospital the authors studied patients with both leukaemia and neutropenia of varying origin. Raised serum muramidase levels in acute myelomonocytic and some other myeloid leukaemias were confirmed. Among 27 patients with neutropenia studied to date, correlations have been made between the following parameters : serum and marrow muramidasc levels, clinical features, marrow and blood counts and morphology. A close correlation was demonstrated between levels of serum muramidase and marrow cellularity. All patients with a low serum muramidase had hypocellular marrow, while 14 of the 19 patients with a normal or elevated muramidase had a marrow which was either normo-or hypercellular. A highly significant correlation was also observed between serum muramidase levels and spleen size : all patients with splenomegaly had either normal or elevated serum muramidase levels irrespective of marrow cellularity. The ratio of marrow serum muramidase, normally 1.5-3 1, was also determined. The ratio was found to be elevated in 5 patients, 4 of whom showed clear morphological evidence of disturbed myeloid, proliferation. It was concluded that serum and marrow muramidase estimations may help in separating neutropenias due to decreased myelopoiesis from those caused by excessive granulocyte destruction.