The authors have used a quantitative immunoblotting technique to analyse the antibody repertoire of IgM in cord blood and in the serum of young children, young adult males and aged males directed towards antigens in homologous tissues utilized as sources of self antigens. The reactivities of IgM with self antigens exhibited striking homogeneity and invariance among newborns. Self‐reactive IgM repertoires of children, young adults and aged males were markedly conserved among individuals and comprised most of the anti‐self reactivities that prevailed in neonates. Reactivities of IgM with bacterial antigens showed a high degree of homogeneity among newborns but were more diverse in children, young adults and elderly individuals. Diversity of IgM reactivities with self and non‐self antigens did not vary significantly with aging. Principal component analysis (PCA) and linear discriminant analysis (LDA) of the data discriminated between self‐reactive IgM repertoires of newborns and children, but failed to discriminate between repertoires of children, young adults and aged males. The data indicate that the self‐reactive antibody repertoire of IgM differentiates during the first years of life and remains relatively constant thereafter.