Histochemical and immunohistochemical studies performed in only a few cases of sinus histiocytosis with massive lymphoadenopathy (SHML) indicated that SHML cells belong to the macrophage--histiocyte system, though their exact origin is still uncertain. We analyzed the morphological, antigenic and enzymatic characteristics of the histiocyte-like cells in one paediatric case of SHML (also named Rosai-Dorfman disease). The SHML cells expressed the S-100 protein, lectins concanavalin A, peanut agglutinin and monocyte-macrophage related antigens CD 11c, CD 14, CD 33, CD 68 and LN 5. Reactivity with other anti-macrophage antibodies (MAC387, lysozyme, alpha-1 anti-chymotrypsin) was variable. The CD1a antigen was present only in scattered cells, whereas HLA-DR and the HLA-DR associated invariant chain were absent. Cytochemistry demonstrated an intense activity of acid phosphatase and non specific esterase of SHML cells. A large amount of medium sized mononuclear cells were located in the sinuses and intersinusoidal tissue. Our findings suggest that SHML cells have intermediate features between phagocytes and Langerhans cells/interdigitating reticulum cells. The heterogeneity of marker expression on SHML cells might be related to the local content of factors (e.g., cytokines), capable of modulating the phenotype of monocyted and derived cells.