The ontogeny of immune responsiveness, as assayed by antibody formation in vitro, of mouse spleen lymphocytes to thymus-independent antigens is reviewed. Responsiveness to trinitrophenyl (TNP)-lipopolysaccharide and TNP-Brucella abortus appear soon after birth and one to two weeks before TNP-Ficoll or capsular polysaccharide of Streptococcus pneumoniae (SSS-III) elicits significant antibody formation. This hierarchy of responsiveness to antigens is also apparent in the CBA/N mutant mouse strain, which has a bone marrow-derived (B-) cell maturation arrest and fails to respond to either TNP-ficoll or SSS-III. These findings are interpreted to suggest sequential maturation of different populations or lines of B-lymphocytes, each of which can respond to a defined class of thymus-independent antigens. The implication for vaccine use in humans is that a late-appearing subclass of B-cells may be required for adequate immune responses to polyaccharide antigens.