Immunoglobulin and T cell receptor gene rearrangement and expression were investigated within B cell acute lymphoblastic leukemias (ALLs) that are thought to be representative of the early stages of normal human B cell development. Although the immunoglobulin heavy chain locus is rearranged in leukemic cells from all of these patients, only cells from pre-B ALL patients express mu chain protein. We have shown, however, that immunoglobulin heavy chain transcripts are expressed at levels similar to those found in normal B cells in all of these leukemic cell types and, in addition, some of the leukemias with a more mature phenotype also express immunoglobulin light chain mRNA. In contrast, leukemic cells which had rearranged T cell receptor gamma chain genes were not found to express gamma chain transcripts, indicating that the transcription of immunoglobulin and T cell receptor genes in these cells may be regulated in a lineage-specific manner.