The murine pre-B cell line R2-bfl, which can be induced to differentiate in vitro, was used to study germline transcription of variable regions of the light chain loci. RNA from these cells was subjected to a 3'-RACE and germline transcripts from 17 individual Vkappa gene segments belonging to 12 Vkappa families were characterized. Germline transcripts of all three Vlambda regions were similarly analyzed. The synchronous differentiation of R2-bfl cells was then used to investigate the order of appearance of germline transcripts of the V and JC clusters of both light chain loci. This was taken as indicator for accessibility of a particular locus to rearrangement. Germline transcripts of the JCkappa cluster and the Vkappa family most proximal to JCkappa was detectable already at day 0, while transcripts of the most distal Vkappa family became apparent after initiation of differentiation at day 1. Transcripts of the JClambda cluster could be found at day 2, whereas transcripts of the Vlambda region were already present at day 1. Thus, the lambda locus becomes accessible to rearrangement later during development than kappa, confirming and extending our previous findings. The V and JC clusters open at the same stage of development although slight asynchronicities were found for the Vlambda and the distal Vkappa gene segments.