Recently, great interest has been shown in the histological identification of small cell tumours of childhood–nephroblastoma (Wilms tumour), neuroblastoma, rhabdomyosarcoma and Ewing's sarcoma–using immunohistochemical methods. However, several antigens operationally specific for leucocyte typing in blood and marrow are also expressed on cells of epithelial and neural origin. We undertook phenotypic characterization of 17 non-haemopoietic small cell tumours of childhood using a panel of 30 monoclonal antibodies to leucocyte, epithelial and cytoskeletal antigens using a sensitive alkaline phosphatase–anti-alkaline phosphatase technique on cryostat sections of fresh tumour. Our results demonstrated frequent expression of the leucocyte-associated antigens CD 10 (CALLA). CD9 (p24) and CDw32 (FcRII) in these small cell tumours and occasional expression of MHC class II (HLA-DR) and HNK-1 antigens. However, the leucocyte-associated antigens CD45 (leucocyte common), CD22 (pan B-cell), CD11b (C3bi receptor), CD15 (Lewisx) or CDw42 (platelet gp Ib) were not detected on any tumour. Aberrant expression of desmin, neurofilament and UJ13A antigen was found in nephroblastoma and of epithelial-associated markers (CIBr17 and 43–9F) in neuroblastoma. Our results also demonstrated broad reactivity in frozen section with two monoclonal antibodies specific for melanoma (NKI/C-3) or epithelial cells (OM-1) in paraffin sections. Hence, it is necessary to include monoclonal antibodies to CD45 and pan-epithelial antigens, e.g. LP34 (cytokeratin) or HEA125 for the precise immunohistochemical identification of small round cell malignancies of childhood.