A cell transfer assay system was developed to study the precursors of bone marrow-associated (B) lymphocytes in the adult mouse. The rationale of the assay is to inject into irradiated mice a cell suspension depleted of B lymphocytes, to wait a period of time to let precursor cells differentiate to B lymphocytes, then to correlate the number of B cells present in the recipient mice with the number of precursor cells injected. The assay as described was shown to be linear in the range of 10 5 –3 x 10 6 fractionated bone marrow cells. Kinetic studies indicated that precursor cells start producing detectable numbers of B cells within 3 days after transplantation; B cell activity then increases with a doubling time of 24 hr. Physical characterization of that precursor cell has shown that it is lighter and sediments faster than small lymphocytes. Precursor cells were found in bone marrow and spleen but could not be detected in peripheral lymph nodes. Results of physical analysis also indicate that the precursors of B lymphocytes described here may not be pluripotent stem cells for the immune system.