By means of an indirect histochemical technique, the intracellular lysozyme of the formed elements of the peripheral blood and bone marrow was estimated. Evidence is presented that monocytes, as well as mature neutrophils and their precursors extending back to the progranulocyte, contain significant amounts of this enzyme. A rare mature eosinophil demonstrated a trace of lysozyme activity. There was no evidence of lysozyme activity in basophils, erythrocytes, megakaryocytes, platelets, plasma cells, tissue mast cells or bone marrow reticuloendothelial cells.