The primary in vitro antibody response of neonatal spleen cells to three thymic independent antigens has been examined. The time of onset of responsiveness to TNP-Brucella abortus and TNP-lipopolysaccharide was significantly earlier than the onset of responsiveness to TNP-Ficoll. This ontologic sequence was not affected by T cell depletion or antigen presentation on adult macrophages. In neonatal mice bearing the X-linked CBA/N defect, the response to TNP-Brucella abortus and TNP-lipopolysaccharide was much delayed and no response to TNP-Ficoll developed. We conclude that different thymic independent antigens address different subpopulations of B cells, one of which appears earlier in ontogeny than the other.