Heavy and light chain variable region nucleotide sequences were derived from 6 human hybridoma antibodies which bear characteristics of antibodies associated with the phospholipid antibody syndrome. All antibodies originated from non-autoimmune individuals and were polyspecific. Four of these reacted with cardiolipin (and other antigens) and three carried the H3 idiotype which is expressed on a high percentage of disease-associated anti-cardiolipin antibodies. This idiotype was localized to the lambda light chain of the H3 monoclonal antibody and found on two other antibodies which like H3 expressed V lambda 4 or the related V lambda 3 subgroup light chains. The H3 idiotype however did not define these subgroups nor was it required or sufficient for anti-cardiolipin activity. Anti-cardiolipin binding was found in VH1, VH3 and VH4 heavy chain families and in a V kappa 1 light chain. The D region was diverse in both length and gene usage. Although all cardiolipin binding antibodies showed little deviation from germline variable (V) gene sequences, where mutations occurred they tended to be replacement mutations and clustered in complementarity determining regions (CDR) suggesting these B cells were derived from antigen-driven responses. These results from our panel of hybridomas and their comparison to other human antibodies provide extensive information on the diversity of genetic elements which can be used by cardiolipin-binding antibodies. We also show gene sequences which encode the disease-associated H3 idiotype and its location on lambda light chains, which imply that some labda light chains may be preferentially utilized in auto-reactive hybridomas.