L1 is an approximately 36-kd protein present in virtually all resting peripheral blood neutrophils and monocytes. It is particularly well preserved in formalin-fixed and paraffin-embedded routine material. In a recent immunohistochemical study, the authors showed that L1 has a restricted distribution within the monocyte-derived cell lineage, being mainly confined to reactive histiocytes (infiltrating macrophages). A protein sharing physicochemical and antigenic properties with L1 was identified in extracts of epidermal scales obtained from patients with psoriasis. This epithelial L1 was generally not expressed by normal epidermis, but its production was abundant in several skin diseases. Moreover, mucosal squamous epithelium normally expressed L1. No other epithelia showed signs of L1 production, although occasional patchy uptake was indicated, particularly in kidney tubular epithelium.