The ontogeny of the ability of B lymphocytes to produce an antihapten response which is heterogeneous with respect to affinity for the antigenic determinant was studied in a cell transfer system. The heterogeneity of affinity of the immune response of lethally irradiated mice reconstituted with syngeneic, adult thymus cells and fetal or neonatal tissues as a source of B lymphocytes was studied. It was found that B cells from 17 day fetal liver or neonatal liver are highly restricted with respect to heterogeneity of affinity as compared with adult spleen or bone marrow. The B-cell population achieves an adult character with respect to heterogeneity of affinity by 2 wk of age. The peripheral lymphoid tissues (spleen) appear to mature in this respect more rapidly than do central lymphoid tissues (bone marrow). Spleens from 10-day old donors behave in an adult, heterogeneous manner while bone marrow from the same donors exhibit a marked restriction in heterogeneity of affinity. Germfree mice produce an immune response which is indistinguishable from conventionally reared adult animals with respect to heterogeneity of affinity. The earlier appearance of the ability to transfer a heterogeneous immune response in spleen as compared with bone marrow suggests that the increasing heterogeneity of the B-lymphocyte population which occurs between birth and 2 wk of age is the result of a differentiation event and not of a somatic mutation or recombination event
