Lymphocytotoxic autoantibodies are commonly found in sera from kidney dialysis patients, where they interfere with interpretation of the crossmatch test. We have produced a monoclonal lymphocytotoxic autoantibody by EBV transformation of PBLs from a dialysis patient, followed by fusion of the transformed cells with a heteromyeloma. The autoantibody derived, called FWE, was IgM kappa class and its pattern of reactivity against T and B lymphocytes, cells from patients with chronic lymphocytic leukemia and K562, was similar to that described for lymphocytotoxic autoantibodies found in sera. It was absorbed by fetal but not adult human erythrocytes, suggesting the antigenic determinant might be the blood group antigen i. cDNA encoding the variable domain of FWE was amplified by polymerase chain reaction, cloned into the vector M13 mp18, and sequenced. The variable region of the kappa light chain (V kappa) was 98.8% identical over a 260-bp stretch with a known germ line sequence and the junctional (J kappa) region was identical over 16 bp with the germ line sequence J kappa 2. The variable region of the heavy chain (VH) was 99.3% identical over a 268-bp overlap with the germ line gene VH4.21, a member of the VHIV family, and the junctional region of the heavy chain (JH) was identical with the germ line JH gene JH5 over 46 bp, with a truncated 5' end. The diversity region was not identified. These data suggest that the genes required to produce human lymphocytotoxic autoantibodies are encoded within the germ line and, therefore, that all dialysis patients may be able to produce them under certain circumstances.