A panel of monoclonal antibodies including antibodies against neuroectodermal antigens, (UJ13A, UJ127.11, UJ181.4), leukocytes (2D1), intermediate filament antigens cytokeratin (LE61), Vimentin, Desmin (Labsystems, Helsinki, Finland), myoglobin, and neurofilament (155) antigens were assessed for their use as an adjunct to light microscopy in pediatric pathology, with particular emphasis on the "small round cell" tumors. One hundred thirty-four tumors were studied using immunofluorescence and immunoperoxidase techniques. The differentiation of neuroblastoma from lymphoma proved to have a clear-cut immunologic profile, as did the rhabdomyosarcomas, which showed consistent positivity with neuroectodermal antibody UJ13A and in positive binding with antidesmin. Ewing's sarcoma did not give a clear immunohistologic pattern with these antibodies. This panel was shown to have been a valuable aid to diagnosis in 12% of cases studied. The future use of such a panel for routine diagnostic use is discussed, but it is emphasized that the binding pattern of these tumors is often heterogeneous, and examination in conjunction with conventional histology is essential if the correct conclusions are to be made.