The (B-cell) repertoire responsive to the DNP and TNP haptenic determinants in BALB/c neonates was analyzed in terms of the specificity of stimulation of neonatal B cells as well as the diversity of specificities available in neonatal populations. The results indicate that the parameters of stimulation of neonatal B cells are similar to those of nonimmune adults, particularly in the exquisitely specific stimulatory process which readily discriminates between haptens as closely related as 2,4-dinitrophenyl (DNP) and 2,4,6-trinitrophenyl (TNP). The clonotypes of monoclonal anti-DNP and anti-TNP antibodies derived from isolated neonatal BALB/c splenic B cells in fragment culture were analyzed by isoelectric focusing. During the first 4 days of neonatal life almost all of the anti-DNP-specific clones were of clonotypes displaying IgM antibodies with pI's of 5.05, 5.25, or 5.55. These could be distinguished from clonotypes responding to TNP which were also predominantly of three distinct pI's, 5.00, 5.15 or 5.40. These clonotypes, which represent the vast majority of the DNP- and TNP-specific antibody capability during the first 4 days of life, represented less than half of the clones by day 6 and were a small minority by day 9. The observation that individual 1--4-day-old donors had many B cells representative of a given predominant clonotype is evidence for cellular precommitment of specificity and indicates that clones of precommitted B cells exist as the products of normal, antigen-independent, generative processes. The observation of frequently recurring clonotypes in inbred neonates attests to the "germ line" origin of these clonotypes; however, variance in the occurrence of these clonotypes from donor to donor implies a random element in their expression. The finding that several clonotypes occur repeatedly in high numbers early in neonatal development, while other clonotypes occur only sporadically at early times, has been interpreted as a reflection of a sequential ontogenic expression of clonotypes. Thus the DNP- and TNP-specific clonotypes which predominate in neonates may be seen as representative of a total of 5,000-10,000 clonotypes which are expressed as early as the 15th to 17th day of gestation while most clonotypes appear after the 18th day of gestation.

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