Human hybridomas have been produced by fusing peripheral blood lymphocytes from patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) with the GM 4672 human cell line. 262 hybridoma clones from the fusions of four RA and five SLE patients were screened for binding to denatured DNA (dDNA), native DNA, and the Fc fragment of human IgG (HIgG). Of the 17 hybridoma antibodies (nine RA, eight SLE) selected for strong binding to denatured DNA, Fc, or both, five reacted with dDNA only, one with Fc only, and eight with both dDNA and Fc. Hybridoma supernatants exhibiting dual reactivity were absorbed over HIgG and bovine serum albumin (BSA)-Sepharose immunoabsorbent columns. The reactivities to both DNA and HIgG were completely removed by the HIgG column but unaffected by passage over the BSA column, and both DNA binding and rheumatoid factor activities were recovered in the acid eluates from the Sepharose-IgG column. The binding of dual reactive hybridoma autoantibodies to the Fc fragment of HIgG was specifically competed by dDNA and HIgG, providing additional evidence that one antibody may be capable of reacting both as a rheumatoid factor and as an anti-DNA antibody.