B cell generation and immunoglobulin (Ig) diversity in mice is compromised with aging. Our recent work sought to understand mechanism(s) that contribute to reduced B cell production in aged mice. Using in vivo labeling, we found that reduction in marrow pre-B cells reflects increased attrition during passage from the pro-B to pre-B cell pool. Analyses of reciprocal bone marrow (BM) chimeras reveal that the production rates of pre-B cells are controlled primarily by microenvironmental factors, rather than intrinsic events. To understand changes in pro-B cells that could diminish production of pre-B cells, we evaluated rag2 expression and V(D)J recombinase activity in pro-B cells at the single cell level. The percentage of pro-B cells that express rag2 is reduced in aged mice and is correlated with both a loss of V(D)J recombinase activity in pro-B cells and reduced numbers of pre-B cells. Reciprocal BM chimeras revealed that the aged microenvironment also determines rag2 expression and recombinase activity in pro-B cells. These observations suggest that extrinsic factors in the BM that decline with age are largely responsible for less efficient V(D)J recombination in pro-B cells and diminished progression to the pre-B cell stage. These extrinsic factors may include cytokines and chemokines derived from BM stromal cells that are essential to the development of B cell precursors. The changes during aging within the BM hematopoietic microenvironment most likely are linked to the physiology of aging bone. Bone degrades with age (osteoporosis) due to decreased formation of new bone by osteoblasts. Marrow stem cells (MSC) are considered the progenitor of both adipocytes, osteoblasts and hematopoietic stromal cells and a controlled reciprocal regulation exists of osteoblast versus adipocyte differentiation; with age adipocytes increase, and osteoblast decrease. It is possible that stromal cell generation from MSC is compromised during aging. Currently, understanding of BM microenvironmental factors that regulate rag gene expression is very limited. However, as early progenitors differentiate, it is increasing clear that a limited set of transcription factors (e.g. ikaros, PU.1, E2A, EBF, pax5) regulate B-lineage specific genes, and that expression and stability of these factors is responsive to the microenvironment. Current and future work by several groups will strive to understand mechanisms that regulate these factors and how aging impacts these regulatory circuits.