The origin of the B cells of splenic marginal zones was studied using transfer experiments in rats depleted of marginal-zone cells. Cyclophosphamide given as a single dose of 500 mg/m2 was used to deplete the marginal zones. Approximately 90% depletion was still apparent 10 days after treatment. Fetal liver cells did not induce rapid repopulation of the marginal zone. Also bone marrow cells from rats depleted of recirculating lymphocytes were inefficient in this respect. Conversely, thoracic duct lymphocytes and bone marrow cells from normal rats were efficient at restoring marginal-zone cell numbers in cyclophosphamide-treated rats. Thoracic duct cells passaged through an irradiated intermediate host and collected from that host's thoracic duct were also efficient at achieving marginal-zone reconstitution. In rats receiving 1000 rd whole body irradiation, which were protected with fetal liver cell transfer, marginal zones did repopulate at about 3 weeks. It is concluded that marginal-zone B cells, after leaving primary lymphoid organs, enter the recirculating pool for a period of at least several days before settling in the marginal zone. The turnover rate of marginal-zone cells was assessed using tritiated thymidine infusion. Most marginal-zone cells were not labeled after 5 days continuous labeling suggesting that the marginal-zone B cells are not rapidly dividing.