The recombination activating gene-1 (RAG-1), which is required for immunoglobulin (Ig) gene rearrangement, is expressed in murine B-lymphoid precursors but not in mature B lymphocytes. In order to characterize the temporal relationship of RAG-1 expression to other markers of human B-lymphoid differentiation [cell surface antigens, terminal deoxynucleotidyl transferase (TdT), Ig gene rearrangements], RAG-1 expression was studied in a group of B lineage childhood acute lymphoblastic leukemia (ALL). ALL cells from 21 patients were grouped into three developmentally related phenotypes based on the expression of the differentiation antigens CD19, CD10, and CD20. All 21 leukemias were surface Ig (slg) negative. There were leukemias representing each developmental stage of Ig gene rearrangement. RAG-1 was expressed in 20 of 21 B-lineage ALL, including leukemic cells from each stage of differentiation, as defined by immunophenotype and IgH and IgL gene rearrangement status. RAG-1 was expressed in slg- ALL, regardless of the Ig heavy chain (IgH) or Ig light chain (IgL) gene configuration. RAG-1 was not expressed in two Burkitt lymphomas and Burkitt lymphoma cell lines with slg+ mature B-lymphocyte phenotype. In two cases, RAG-1 was expressed in TdT-negative ALL; conversely TdT was expressed in the one RAG-1 negative ALL. These results suggest that RAG-1 in B-lineage ALL is expressed at all phenotypic and genotypic developmental stages preceding surface immunoglobulin expression, and that TdT and RAG-1 may be regulated by different mechanisms.