The monoclonal anti-pan-T cell antibody LAU-A1 stained neoplastic T cells arrested at different levels of maturation from all 21 children with T cell malignancies examined. Particularly in 7 patients with immature T cell neoplasia staining for LAU-A1 facilitated the recognition of a T cell origin of the malignant cells. Only 40% of these immature T cell malignancies were associated with an anterior mediastinal mass. A subdivision of T cell neoplasia into 4 differentiation-related subgroups did not permit to make predictions regarding the patients’ survival. Despite the rather uniform clinical presentation the immunological phenotypes of tumor cells in 14 children with B cell non-Hodgkin’s lymphoma (B NHL) were heterogeneous. Tumor cells lacked surface immunoglobulins (2 patients), expressed IgM only (7 patients), IgM and IgD (3 patients) or IgM, IgD and IgA (2 patients). Regardless of surface immunoglobulin expression anti-Y 29/55 stained practically all recognizable tumor cells of all B NHL examined. No correlation was found between the number of heavy-chain isotypes expressed on tumor cells and the survival of the patients. The only long-term survivors were 3 children transplanted with autologous bone marrow which had been purged in vitro with anti-Y 29/55 and complement.