Immunoglobulin (Ig) variable (V) region genes are assembled in precursor B (pre-B) lymphocytes from multiple germline segments. The heavy-chain V-region gene is composed of variable (VH), diversity (D) and joining (JH) segments; kappa (K) and lambda (lambda) light-chain V-region genes have analogous VL and JL segments. Assembly of Ig V-gene segments, as well as those of the highly related T-cell receptor, is regulated at several levels and shows both stage and tissue specificity; for example Ig heavy-chain V-gene assembly precedes that of Ig light chains during B-cell differentiation. Joining of all classes of V-gene segments involves conserved recognition sequences that are probably targets for a common recombinase. Evidence has been presented suggesting that rearrangement of specific classes of segments is regulated by modulation of their accessibility to the recombinase. To elucidate mechanisms which control V-region gene assembly, we have investigated the effect of flanking gene expression on the frequency at which introduced V-gene segments are assembled in pre-B cell lines. Our findings suggest that transcription may play a direct role in the regulation of immunoglobulin V-gene assembly.