The nucleotide sequences of heavy and light chains from 10 monoclonal IgM anti-IgG1 (RF) antibodies were determined and reported here as translated amino acid sequences. Only three families of VK light chains were used in these antibodies: VK1 (two examples), VK8 (three examples), and VK19 (four examples). This represents a significant nonrandom selection of light chains. In contrast, all other variable region gene segments (i.e., VH, DH, JH, and JK) were used in a pattern consistent with random selection from the available pool of germline genes. In two cases, the same anti-IgG1 specificity was generated by a combination of very homologous light chains with unrelated heavy chains. We infer from this that the light chain is the segment used by these antibodies to bind IgG1. The nature of these sequences provides an explanation for the curious observation that as many as 15% of splenic B cells in normal mice may be expressing IgM anti-IgG; if, as our data suggest, certain light chains in combination with many different heavy chains can be used in assembling the anti-IgG specificity, then, because of combinatorial association in which the heavy chain is not relevant for specificity, the fraction of IgM-producing B cells expressing these light chains should approximate the fraction of B cells making IgM anti-IgG. We calculate, based on data presented in several other studies, that 5-17% of B cells express one of the VK types observed in monoclonal RF. This agrees well with estimates for the number of B cells making IgM anti-IgG. In addition, our findings could rule out other explanations of the high percentage of B cells making RF, such as constant stimulation by antigen or presence of numerous antigenic epitopes since it was shown that IgM anti-IgG1 antibodies are not somatically mutated and that they are structurally homogeneous. We aligned the VK sequences of the RF in hopes of finding some primary sequence homology between the represented VK families which might point to residues involved in the binding interaction. Although we found no such homology in the hypervariable regions, we did find significant and unexpected homology in the FR2 and FR3 of these light chains. We noted that these regions are exposed in the Ig structure and postulate that they may be involved in a unique type of binding interaction between two Ig family domains, i.e., VK binding to a constant region domain of IgG.