The expression of macrophage antigens KP1, Mac, lysozyme, and alpha-1-antichymotrypsin was investigated on routine paraffin sections from 17 cases of Langerhans' cell histiocytosis (LCH). All the major clinical forms were represented, including single lesions and monosystemic and multisystemic disease. In all the cases, a variable fraction (3-35%) of LCH cells was immunoreactive with KP1 and anti-Mac; the staining pattern was quite typical because the immunoreaction product was often confined to the perinuclear space and the Golgi area. LCH cells containing lysozyme and AACT were detected less frequently; however, in positive cases the percentage of LCH cells immunoreactive for lysozyme and AACT was in the same range as that of KP1-positive cells. On immunostained cytosmears (one case), about 10% of the CD1a-positive cell population was reactive for the macrophage antigens CD14 and PAM-1. No association was noted between the number of KP1-positive cells and the clinical form and/or anatomic site of the lesion. Phagocytic macrophages were significantly and diffusely immunoreactive with KP1 and anti-Mac and for AACT and lysozyme. Multinucleated giant cells with irregular nuclei were frequently observed; these cells were rarely S-100 positive, were consistently stained by KP1 and AACT, and were occasionally anti-Mac positive. The authors' findings suggest that antimacrophage monoclonals, in conjunction with S-100 protein, may represent a useful tool to establish the diagnosis of LCH in paraffin-embedded material.