Regulation of V-(D)-J recombinations that occur in antigen receptor encoding genes remains poorly understood. Recently, two genes, RAG1 and RAG2, that are able to activate rearrangement of synthetic recombination substrates were cloned in mouse and a human gene homologous to RAG1 was described. To define the differentiation stages corresponding to RAG1 and RAG2 RNA expression, we have studied a large number of B- and T-lymphoid neoplasias. First, we show that a human gene homologous to the murine RAG2 is transcribed in humans. Moreover, using a polymerase chain reaction approach, we have shown that RAG are expressed not only in T-cell receptor (TCR)-negative T-cell acute lymphoblastic leukemias (T-ALLs), but also in some cases in which a significant percentage of cells expressed surface TCR. Absence of RAG expression was shown in certain T-ALLs at variable stages of thymic differentiation. Data obtained in B-lineage ALLs show that RAG RNAs are expressed in almost all slg- B-lineage ALLs but are not transcribed in the slg+ B-cell proliferations tested, including Burkitt's ALLs, follicular center cell lymphomas, and chronic leukemias. These findings are consistent with the involvement of RAG in the control of in vivo V- (D)-J recombinations. These findings are also of interest in the delineation of potential regulatory factors acting on RAG transcription and in the understanding of the mechanisms of specific chromosomal abnormalities occurring in immature lymphoid cells.