During B cell development, cell surface molecules are expressed or are down regulated and the pattern of expression of these molecules can identify different stages of differentiation or activation of B cells. These molecules also provide signals to the B cell for further diferentiation or activation. We have been characterizing a monoclonal antibody that recognies an activation induced antigen on B and T lymphocytes. The antigen recognized by this antibody is 16-18 kD. This molecule is upregulated to varying levels on small resting B cells with different stimuli. Using the immature B cell line Wehi 231.4 as a model, we find that the highest expression of our molecule is on apoptotic cells. In addition, stimulation through CD40 on Wehi 231.4 cells as well as small resting B cells downregulates its expression. The ultimate goal of a B cell is to produce antibodies. Wile the mechanism of antibody generation is largely understood, the details of the rearrangement process and the process of "choosing" which gene segments to recombine are not . We are studying a small Vk family, Vk10, which is known to be utilized in a number of different antigenic responses in several different murine inbred strains. Two of the 3 members of this family have been detected in functional antibodies. Sequence analysis of the third Vk10 gene reveals no structural reason preventing the expression of this Vk10 gene. Furthermore, the promoter, splice site and recombination signal sequences are intact. Under conditions where t mRNA or DNA rearrangements of the 2 expressed Vk10 genes could be detected, no mRNA or DNA rearrangements of the 3rd gene were. A comparison between the promoters of an expressed and the non-expressed Vk10 genes shows eqivalentce in their ability to drive expression of a reporter gene in B cells and plasma cells but a significant difference in their efficiencies in pre-B cells, the stage of development when light chains begin to rearrange. Sequences of rare rearrangements of the under utilized gene show a frequent deletion of an invariant proline residue at the VJ junction. It is possible that such a deletion leads to a non-funcitonal antibody. A cell harboring such a deletion would be lost due to apoptosis or could undergo receptor editing and lose this non-productive rearrangement. These data suggest that functional germline genes do not contribute equally to the primary antibody repertoire.