This paper maps the developmental stage reached by B [bone marrow-derived] cells in mouse fetuses of different ages. The maturity of these cells was assessed in a functional way by measuring the time taken for cells to generate antibody-forming progeny when injected into carrier-primed irradiated adult hosts and challenged daily with antigen. A steady uninterrupted progression in the development of B cells occurred between 13 days of gestation and birth. There was no evidence for a stage during which B cells could only be paralyzed and not induced by antigen.