Pathologic interpretation of an osteolytic lesion from the skull of a 13-month-old boy was amplified by histocytochemistry of cells grown in a methylcellulose clonal culture system. Electron microscopy demonstrated the presence of X granules in the cytoplasm of malignant histiocytes, confirming a diagnosis of histiocytosis-X. Freshly fixed tissue containing histiocytosis-X cells and granulocytes showed histiocytosis-X cells that were positive for alpha-naphthyl acetate esterase (non-specific esterase) and negative with naphthol AS-D chloracetate as the esterase substrate (specific esterase). Clonal cell aggregates, harvested after 6 days' growth in culture, showed histiocytosis-X cells that were positive for both the nonspecific and specific esterases. Differences in staining reactions for the histiocytosis-X cells may be explained on the basis of immaturity of the histiocytosis-X cells growing in culture. This interpretation would support their origin from monocytes, monocytic precursors, or a still less differentiated myelomonocytic precursor cell. Furthermore, gel systems of clonally cultured cells appear to provide a useful tool for the growth and analysis of histiocytosis-X cells.