Transfer of peritoneal cells from adult donors into newborn, allotype-congenic mice led to colonization of the recipient mice by donor-derived B lymphocytes expressing the Ly-1 surface marker (Ly-1 B cells). These cells not only persisted in the recipient mice for at least 5 months, but also increased in number. In contrast, bone marrow-derived stem cells did not or scarcely give rise to B cells after intraperitoneal injection into congenic newborn recipients under the same experimental conditions. Approximately half of the IgM in the serum of peritoneal cell-recipients was produced by donor-derived Ly-1 B cells, suggesting that high levels of serum IgM in a normal mouse are produced by this B cell subpopulation. The transferred Ly-1 B cells were able to respond in a normal fashion to alpha (1----3)dextran, but they did not participate in thymus-dependent and -independent (TI II) immune responses to the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP). In neither the immune response to alpha (1----3)dextran nor that to NP were we able to detect an influence of the transferred Ly-1 B cells on the selection of the idiotypic repertoire in the recipient mice.