This study addressed several questions concerning age-related changes in human B lymphopoiesis. The relative abundance of pro-B, pre-B, immature, naive, and mature B cells among the CD19(+) lymphocyte fraction of human bone marrow was found not to change appreciably over the interval between 24 and 88 years of age. Moreover, proliferation of pro-B and large pre-B cells in adult marrow equaled that observed with fetal marrow specimens. Exceptionally low numbers of lymphocyte precursors were found in some marrow samples, and the values obtained were used to determine parameters that best reflect B lymphopoiesis. Cord blood always contained higher incidences of functional precursors than adult cells. However, sorted CD34(+) Lin(-) CD10(+) progenitors from cord blood and adult marrow had equivalent potential for differentiation in culture, and notable age-related changes were found in more primitive subsets. A recently described subset of CD34(+)CD38(-)CD7(+) cord blood cells had no exact counterpart in adult marrow. That is, all adult CD34(+)Lin(-)CD7(+)CD10(-) cells expressed CD38, displayed less CD45RA, and had little B-lineage differentiation potential. The CD7(+) fractions in either site contained progenitors for erythroid and natural killer (NK) lineages, and ones sorted from marrow expressed high levels of transcripts for the CD122 interleukin 2 (IL-2)/IL-15 receptor required by NK-lineage precursors. Dramatic changes in human B lymphopoiesis occur early in life, and more information is required to construct a probable sequence of differentiation events prior to the acquisition of CD10.