The L1 antigen is a highly immunogenic protein of about 36,500 daltons that can be purified from granulocytes with good yield. Immunocytochemistry with a rabbit anti-serum raised against L1 showed it to be present in the cytoplasm of virtually all resting peripheral neutrophils and monocytes. Moreover, immunofluorescence staining demonstrated variable expression of L1 on the plasma membrane of both these cell types, usually along with lysozyme. This indicated that L1 represents a secretory product like lysozyme as their coexpression on the surface of vital cells was contrasted by the absence of lactoferrin. Cytoplasmic L1 was well preserved by both precipitating and cross-linking fixatives, the latter being preferable to avoid leaching out of antigenic material and to obtain good cellular morphology. Thus, fixation for 3 minutes at room temperature in glutaraldehyde (1%)-formaldehyde (3%) afforded excellent immunoperoxidase staining, particularly when a calcium-containing buffer was used. L1 was not found in eosinophilic granulocytes or in resting B- and T-lymphocytes. Neither did blast transformation of lymphocytes seem to induce L1 expression.