In mice, the early antibody repertoire is more autoreactive and multireactive than the adult antibody repertoire. This multireactivity may provide diversity of antigen binding when the antibody repertoire uses fewer Vh genes. The ontogeny of the serum antibody repertoire has not been studied longitudinally in infants. Using a novel Western blotting technique and principal components statistical analyses, we have studied the antibody repertoire from longitudinal samples of paired maternal delivery (n=12), infant cord blood (n=12), 6 mos (n=10) and 9 mos plasma (n=8). When the plasma autoreactive IgM repertoire was tested against Jurkat cell lysate, the maternal repertoire at delivery differs significantly from that of their infants at birth, 6 mos or 9 mos of age within principal components factor 1 (p<0.0001). Autoreactive cord blood IgM differs significantly from 9 mos plasma from the same infants within factor 3 (p<0.01). In contrast, cord blood IgM repertoire against S. sanguis lysate differs significantly from maternal repertoire within factor 1 (p<0.0001), but the infant repertoire becomes more like their mothers over time until the difference disappears by 9 mos of age. These results show that an infant's IgM repertoire changes between delivery and nine months of age, relative to their mothers, with different emerging repertoires against bacterial versus autoantigens. These and other children will be sampled as they grow older to determine when their plasma IgM repertoire begins to resemble adult IgM profiles and to examine parallels in serum IgG and saliva IgA repertoires.