Among all adult T and B cell Ag receptor chains, only Ig light chains lack N regions. It is thought that this is due to the fact that light chain genes rearrange after heavy chain genes, and that terminal deoxynucleotidyl transferase, the enzyme that adds N regions, is not longer expressed at that stage. However, this concept has been challenged recently by the demonstration that 3 to 10% of B cell precursors (CD43+B220+) appear to rearrange their light chains at approximately the same time as they undergo VH-->DJ rearrangements. To examine N region addition in B cell precursors undergoing early kappa-chain rearrangement, we PCR amplified rearranged V kappa 21 genes from the CD43+B220+ bone marrow cells and compared them to sequences obtained from whole bone marrow and spleen. Unexpectedly, all three populations showed approximately 10% N region containing junctions, most consisting of only one N nucleotide. Thus, even the B cell precursors that rearrange light chains at this early stage of development lack much N region diversity. Twelve percent of the sequences unambiguously contained P regions, which were from 1 to 5 nucleotides in length. All but 2 of the 41 productive rearrangements had the commonly observed CDR3 length of nine amino acids. Many (71%) of the sequences were out of frame. CDR3 length was very restricted in nonproductive rearrangements too, and deletion of nucleotides from V kappa and J kappa gene segments was limited. Thus, even at the level of nonproductive rearrangements, junctional diversity is minimal for kappa-chains.