Human B-cell lines were derived by limiting dilutions of Epstein-Barr virus (EBV) transformed peripheral B cells from a patient with an autoantibody against glycoprotein (GP)Ia/IIa, and manifesting defective collagen-induced platelet aggregation and a bleeding problem. Antibody- producing clones were selected for their reactivity with whole platelets or with affinity-purified GPIa/IIa by enzyme-linked immunosorbent assay (ELISA). One of these cell lines, selected for further evaluation, produced an IgM (E3G6) that interfered with platelet aggregation responses. Polymerase chain reaction (PCR) amplifications with two different sets of primers specific for human kappa-chains resulted in the rescue of a unique and identical sequence. The same was true for the mu-chain, from which it was concluded that the cell line was monoclonal. Further analysis showed that the kappa variable domain sequence is similar to the germline gene A30, to 2E7, an anti-GPIIb human autoantibody, and to HF2–1/17, a systemic lupus erythematosus (SLE)-associated broad-specificity human autoantibody. Thus, the specificity of our antibody, E3G6, appears to be determined by the mu-chain, the sequence of which is encoded by a VHIII gene segment strongly homologous to the germline gene DP-77, by a D gene that is not homologous to any of the germline D genes reported to date, and by JH4 gene segment that is germline. All four mutations versus DP- 77 are in CDRs, and result in amino acid substitutions, which implies that E3G6 may have been derived from an antigen-driven response.