The investigators propose to provide more direct information on the specificity of human IgM rheumatoid factors (RFs) and particularly those with heteroclitic reactivity. Thus, the investigators will explore the immunochemical basis for RF anti-Gm(a) specificity in a GM(a) negative RA patient, as well as why GM(a) negative RA patients RF react with GM(g)(+) IgG. Since most human RFs probably react at a relatively small 4-8A0 site between the C-gamma 2 - C-gamma 3 interface, changes in either conformation or selected residues in this region will be studied with particular respect to the individual Gm specificities of RFs isolated from serum by monomeric IgG affinity columns or with high avidity human monoclonal RFs produced by EBV stimulation of CD5 restricted B-cells. Quantitative studies of Gm phenotypes of IgG produced during short-term five to seven day pokeweed cultures of normals and RA patients will be completed using newly developed mouse MAbs to Gm(a) and Gm(f) determinants. These studies will attempt to examine whether lymphocytes from Gm(a) negative RA patients and normal controls will produce small amounts of Gm(a)(+) IgG when cultured in vitro with pokeweed mitogen. In parallel Gm(a) oligonucleotide-specific probes will be used with DNA in Gm(a) negative subjects to determine through polymerase chain reaction amplification the possible occurrence of mutational Gm(a) coding nucleotide sequences occurring in cultured PBMC from normals and RA subjects. Since three histidines are clustered together in residues 433, 435, and 310 at the C-gamma 2 - C-gamma 3 cleft where RF binds, chemical alteration of these histidines using diethylpyrocarbonate, platinum chromophores or spin-labeling will be attempted to study the effects both on Gm-related antigens as well as RF binding. Moreover, myeloma proteins of known Gm phenotypes will be studied after chemical histidine, tyrosine or lysine modifications to determine relative contributions of these residues to conformations or single residues responsible for Gm(a), (b), (g), or (f) allotypic specificities.