This study investigates the presence and characteristics of a basic, cationic protein (CP) in the urine and serum of patients with monocytic and monomyelocytic leukemia. CP was identified through electrophoretic analysis and characterized as a low molecular weight constituent with lysozyme (muramidase) activity. Increased levels of CP-lysozyme were detected in the serum of monocytic leukemias using immunochemical and enzymatic techniques, but not by electrophoresis. The study also examines the clinical and hematologic patterns of ten patients with these leukemias, noting renal and electrolyte abnormalities in six cases. The authors discuss the chemistry and distribution of lysozymes, their role in host defenses, and their association with various disease states, including tuberculosis, renal diseases, and chronic infections. They conclude that CP-lysozyme is likely the normal enzyme that is elaborated in excess in monocytic leukemias, and its urinary excretion is attributed to its small molecular size.This study investigates the presence and characteristics of a basic, cationic protein (CP) in the urine and serum of patients with monocytic and monomyelocytic leukemia. CP was identified through electrophoretic analysis and characterized as a low molecular weight constituent with lysozyme (muramidase) activity. Increased levels of CP-lysozyme were detected in the serum of monocytic leukemias using immunochemical and enzymatic techniques, but not by electrophoresis. The study also examines the clinical and hematologic patterns of ten patients with these leukemias, noting renal and electrolyte abnormalities in six cases. The authors discuss the chemistry and distribution of lysozymes, their role in host defenses, and their association with various disease states, including tuberculosis, renal diseases, and chronic infections. They conclude that CP-lysozyme is likely the normal enzyme that is elaborated in excess in monocytic leukemias, and its urinary excretion is attributed to its small molecular size.