Chicken immunoglobulin lambda light chain rearrangement is regulated by different controlling elements. One negative control element, acting as a strong transcriptional silencer, is located in the V-J intervening sequence which is excised during the rearrangement process. Positive control elements include the V lambda promoter, the enhancer of transcription (3' of C lambda), and one (or two) putative antisilencer element(s) located on each side of the silencer. It is proposed that these antisilencer elements counteract transiently the silencer thus allowing rearrangement of one allele in chicken B cell progenitors. The implications of this regulation for mouse B cell development are discussed.