Certain rheumatoid factors react with both IgG and an antigen associated with cell nuclei.

Rheumatoid factors (RF) were isolated by their affinity for human IgG. RF from three out of twenty-four sera (12%) with a RF-latex titre greater than or equal to 20 and an anti-nuclear antibody (ANA) titre greater than or equal to 250 cross-reacted with nuclei from mouse cells. This antibody which represents an unknown proportion of RF, is designated RF/ANA. Only one of the three sera (Nil) contained large amounts of RF/ANA. A fourth serum (Jos) had 6.6 mg of RF per ml and an ANA-titre of 4096; all the ANA-activity of this serum was absorbed by an IgG-agarose conjugate. Part of the RF-activity of serum Jos was carried by antigenically deficient IgM. The ANA-activity of purified RF/ANA from four sera was inhibited by low concentrations of aggregated human IgG and by higher concentrations of human Fcgamma fragments and non-aggreated mouse anti-DNP antibodies. The data provide strong support for the conclusion that certain antibody molecules can combine with both IgG and an antigen in cell nuclei.