We use techniques of immunogenetics and of molecular biology to study the rabbit immunoglobulins (Igs), and related genes such as the recombination activating genes RAG-1 and RAG-2 which are necessary for gene rearrangements to occur during lymphocyte development. We investigate the development of anatomical sites such as appendix follicles and germinal centers in gut associated lymphoid tissues and the regulated expression and sequence diversification of Ig genes during lymphoid cell development. Membrane IgD has not been definitively identified on rabbit B cells; the genetic region which should contain the delta gene has been cloned, but sequences corresponding to delta heavy chain exons have not been identified. We have identified two surface immunoglobulin complexes on rabbit peripheral blood B cells that possess heavy chains of similar apparent molecular size confirming previous reports of the presence of a second surface receptor expressed on rabbit B cells which may be IgD or an IgM-related molecule. In noncovalent association with one or both of these receptors are several proteins that are components of surface immunoglobulin (mIg) complexes expressed on rabbit B cells. Three proteins (42 kDa, 37kDa, 36kDa) were found in association with IgM expressed on B cells from peripheral blood and spleen. These proteins may be the rabbit homologues of murine Ig-beta (B29), Ig-gamma (truncated B29) and Ig-alpha (mb-1). Lectins are allowing us to dissect rabbit appendix follicles and germinal centers into subcompartments based on local expression of glycosylated cell surface receptors. We found two additional IgM-associated protein complexes containing 36kDa and 34kDa proteins in lysates from individual follicles isolated from the appendix. The appendix may be functioning early in development like the chicken bursa or sheep ileal Peyer's patch. However, the rabbit appendix does not involute but may alter its function and become a secondary lymphoid organ in the adult.