Two monoclonal, natural IgM antibodies derived from normal BALB/c mice were selected on the basis of being idiotypically complementary and functionally connected. Nucleotide sequence analysis of their respective heavy and light chain V regions showed that both of the clones expressed V(H), D, J(H), V(L) and J(L) gene segments of germ-line origin. Furthermore, none of the clones displayed N-sequence additions. These data suggest a germ-line origin of a functional idiotypic network and confirm a minimized contribution of somatic diversification through template-independent addition of N-nucleotides in neonatal B cell repertoires.