The Pediatric Oncology Group has studied 1,367 patients with non-B cell acute lymphocytic leukemia (ALL) of whom 186 (14%) had blasts that reacted with previously well-characterized heteroantisera, recognizing a T-lymphocyte specific surface membrane antigen (PT+). In 87 of these T cell cases, the leukemic cells failed to form at least 20% of sheep erythrocytic rosettes at 4 degrees C. Comparison of clinicopathologic features among PT-, E-PT+, and E+ groups of patients revealed significant differences among them. E-PT+ patients were older than PT- patients, had higher white blood cell counts (WBCs) and were more likely to have a mediastinal mass, and thus contained a higher proportion of poor-risk patients. However, the E-PT+ patients were also significantly different from the more traditionally-defined E+ patients in that they had lower WBCs and hemoglobin levels, and less frequent lymphadenopathy or mediastinal mass. In many respects, then, E-PT+ patients were intermediate in character between PT- and E+ patients. Our findings support the notion that further subclassification of ALL using antibodies recognizing lineage-specific surface determinants will permit recognition of groups of patients with distinct clinicopathologic features that may differ in prognosis or response to therapy. Such classification also focuses future biological studies on the pathogenesis of leukemias to immunologically well-defined subgroups of lymphoid neoplasms.