Intracytoplasmic lysozyme (muramidase) may be readily identified in paraffin sections of tissues fixed in formalin or Zenker's acetic acid and in smears of peripheral blood or bone marrow using an immunoperoxidase technique. Sites of intracellular lysozyme in normal human tissues and in various specimens from patients with myeloproliferative and lymphoproliferative disorders, hairy cell leukemia, granulomatous diseases, toxoplasmic lymphadenitis, and other pathologic processes were defined by this method. Intracellular lysozyme was demonstrated in mature and immature neutrophilic and eosinophilic myeloid cells, in monocytic cells, and in some types of histiocytes and had a limited distribution in normal tissues. The neoplastic cells of hairy cell leukemia were devoid of intracytoplasmic lysozyme. Identification of intracellular lysozyme, as determined by the immunoperoxidase technique, was compared with various cytochemical methods, particularly chloroacetate esterase and alpha-naphthyl butyrate esterase studies, for detection and characterization of myeloid cells, monocytes, and histiocytes.