Evidence for anti-self-reactivity in the preimmune B cell repertoire has been well documented. The present study aimed to determine the frequency of antibody-forming cell precursors in this repertoire whose Ig V regions impart reactivity to "self" or autologous Ag. Clones were activated in vitro with LPS and their secreted IgM antibody was assayed for reactivity by direct binding to cell surface or intracellular Ag. An IL-4-containing lymphokine mixture was added to the clonal cultures to induce the secretion of IgG1. The reactivity of secreted IgG1 with Ag would more closely resemble the binding required to activate B cells through their monomeric surface IgM and/or IgD. The results indicate a high frequency of precursors secreting IgM with reactivity to intracellular Ag, namely 1 in 37 +/- 6 B cells, with a marked paucity of response to cell surface molecules. The repertoire was markedly deficient in precursors secreting IgG1 able to bind to intracellular Ag, with only one clone detected by the screening of 3.0 x 10(6) spleen cells. No positives were detected for cell surface Ag. This suggested that the frequency of clones in the preimmune repertoire that express IgR with sufficient affinity to bind "self" molecules must be very low.