To elucidate the molecular basis for the ability of antibodies encoded by the human VH26 heavy-chain variable region gene to react with diverse antigens, we have generated 34 hybridomas secreting chimaeric monoclonal antibodies (human mu heavy chain/mouse light chains) from transgenic mice. The transgenic mice carry an immunoglobulin minilocus containing the human VH26 gene, human DH and JH gene segments, and genes encoding the human C mu region. The minilocus in these animals undergoes functional rearrangement resulting in the production of chimaeric antibodies in which human mu heavy chains utilizing the VH26 gene are paired with mouse kappa or lambda light chains. The hybridomas described in this study were generated from naïve animals and were selected solely on the basis of human mu-chain expression. The antibodies described have covalently attached mouse light chains and are multimeric in structure. The binding properties of the antibodies were examined using a panel of both self- and foreign antigens using enzyme-linked immunosorbent assays, agglutination or radio-immunoprecipitation assays and immunofluorescence. Chimaeric immunoglobulins from 21 of the 34 hybridoma clones (61.7%) reacted with one or more antigens, of which 13 (38.2%) reacted with more than two antigens. These studies demonstrate that the VH26 gene, in combination with human DH and JH gene segments, and mouse light-chain genes, is able to encode antibodies with a wide range of ligand-binding specificities. These findings have important implications in the context of the possible origins of autoantibodies encoded by VH26 which may play a role in the pathogenesis of a number of autoimmune conditions.