The ability of antibody-forming cell precursors (B cells) obtained during murine development to participate in the splenic focus response to a hapten-protein conjugate was determined by the technique of adoptive cell transfer to irradiated, syngeneic, carrier-primed recipient mice. Spleen fragments from recipients of fetal liver, fetal spleen, or neonatal spleen cells produced anti-hapten antibody when exposed in in vitro primary organ culture to dinitrophenylated hemocyanin (DNP-Hy). The number of antibody-producing fragments (splenic foci) obtained was linearly related to the number of nucleated spleen cells injected, and did not vary significantly for neonatal spleen cell populations obtained during the 1st week of postnatal life. Although neonatal foci released less antibody than that reported for adult foci (1), the dose dependence of antigenic stimulation is comparable for both neonatal and adult primary precursor cells, implying comparable mechanisms for antigenic recognition and stimulation. The frequency of precursor cells in the neonatal spleen for the haptenic determinant DNP is only slightly lower than the reported frequency (1) of adult precursor cells of the same specificity. This finding is discussed in terms of the relevance of antigenic selection to the generation of diversity.

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