The transit of immature to mature sIgM⁺ B cells, the life span, maturation kinetics and response to polyclonal activators have been analyzed with the help of a new mAb (493), that distinguishes immature, 493⁺ from mature, 493⁻ B cells in a variety of mouse strains tested. Analysis of the turnover of immature 493⁺ B cells by bromodeoxyuridine (BrdU) labeling kinetics indicate that only 10 – 20 % of the cells reach the spleen as immature 493⁺ cells. The life span of 493⁺ B cells in bone marrow and spleen is around 4 days. BrdU chase experiments show that most of the immature cells in spleen enter the pool of mature, 493⁻ B cells where they gain a longer life span of 15 – 20 weeks. Immature and mature B cells respond equally well to LPS stimulation; anti‐CD40, however, stimulates mature B cells better than immature B cells. IgM cross‐linking of mature B cells results in proliferation, while it induces apoptosis in immature B cells. This apoptosis of immature cells can be inhibited by co‐stimulation with anti‐CD40 or by overexpression of bcl‐2. We speculate that Ig receptor ligand‐mediated apoptosis (negative selection) plays a major role in the transit of immature B cells from bone marrow to spleen, but only a minor role in the transit from immature B cells to mature B cells in the spleen.