A cohort of newly produced virgin B cells was followed from the marrow to the spleen of non-immunized clean rats, which showed minimal antigen-driven proliferation of B cells in their spleens. The progenitors of this cohort of virgin cells were labeled in vivo over 12 h with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdUrd) and their non proliferating progeny left the marrow 2-3 days later. This coincided with the arrival of labeled B cells in the red pulp and T zones of the spleen. These appear to be short-lived as few remained a week after the label was given. The short-lived newly produced virgin B cells can only comprise a minority of splenic B cells, for it is shown that only 20% of splenic B cells are found in the red pulp and T zone. It is calculated that newly produced virgin B cells are likely to make up between 5% and 10% of splenic B cells. In the marginal zones and follicular mantle respective medians of 3.3% and 1.8% were already labeled at 1 day from the start of the BrdUrd pulse. The appearance of these cells seems likely to result from antigen-driven B cell proliferation outside the marrow, for labeled virgin B cells have not started to leave the marrow at this stage. During day 2 and 3 the proportion of labeled follicular mantle B cells rose to 3.4%, which might in part reflect the recruitment of newly produced virgin B cells to the pool of recirculating follicular B cells. After day 3 in the follicles and day 1 in the marginal zones the proportion of labeled cells did not vary significantly through day 7. This appears to confirm the comparative longevity of the cells in these zones, which contain 80% of the non-proliferating splenic B cells of adult rats.