Fifty paraffin-embedded medulloblastomas (31 in children and 19 in adults) were reacted with a panel of ten antibodies to glial, neuronal, mesodermal and epithelial antigens. The tumours were divided according to their histological features into three groups: classic, desmoplastic and highly vascular. Reactivity for glial fibrillary acidic protein was observed in 20 cases. Forty tumours reacted with PGP9.5 (neuronal marker) in clusters of poorly differentiated cells, cell cords and some scattered cells. Cells forming rosettes were mostly negative except for slight central reactivity. Eight of the 40 tumours contained neurofilaments. In scattered cells somatic reactivity for vimentin was found in 14 tumours. Ten cases showed positivity for S-100 with a nuclear and perinuclear pattern. No difference in reactivity in relation with age was observed. Desmoplastic medulloblastomas showed less reactivity for glial and neural markers. It was concluded that medulloblastoma shows degrees of differentiation as evidenced by the expression of various proteins. Differentiation occurs along two lines: glial and/or neuronal. Most tumours also contain a component of poorly differentiated cells which may differentiate into one of these two lines or act as primarily stem cells.