Cell activation antigen expression, because it is related to cell proliferation, may offer useful information about tumor characteristics and treatment outcome. To assess the clinical utility of assays for the plasmocyte activation antigen CD38 (T10) and the thymocyte activation antigen CD71 (transferrin receptor; T9) in childhood acute lymphoblastic leukemia (ALL), we reviewed the presenting features and clinical outcomes of 325 ALL patients treated on a single protocol at St Jude Children's Research Hospital. T-cell ALL cases were more likely than B-lineage ALL cases to express CD38 (100% versus 90.5%, p < 0.01) and CD71 (92% versus 32%, p < 0.01). However, expression of these antigens was not related to any clinical or biologic feature within the immunophenotypic subgroups. More importantly, event-free survival did not differ significantly between CD38+ and CD38- cases or between CD71+ and CD71- cases in the study group as a whole or in immunophenotypic subgroups. Studies of CD38 and CD71 expression in childhood ALL provide no clinically useful information beyond that obtained from standard immunophenotyping studies.