A nuelear SS-B antigen was isolated from a saline extract of acetone powder of rabbit thymus by precipitation with ammonium sulphate, affinity chromatography with Blue Sepharose CL-6B, and preparative agarose gel electrophoresis. The mol. wt of the antigen was 68,000. Its electrophoretic mobilily was similar to that of pro-albumin, and the iso-electric point was around pH 4.0. The main amino acids of the antigen were gluiamie acid, leucine, lysine and alanine. Both histidine and tyrosine were also found. The purified antigen precipitated with anti-SS-H sera but not with any other reference antisera. It resembled La and Ha antigens in susceptibility to proteolytic and nucleolytic enzymes and to heat. The purified SS-B antigen, however, had a higher molecular weight than did the Ha and La antigens. The molecule eould not be split inlo subunits with mercaptoethanol or acid. Counter-electrophoresis showed antibodies to the SS-B antigen in sera from patients wilh rheumatic diseases, including rheumatoid arthritis, systemic lupus erythematosus and Sjögren's sjndrome, but not in any of the control sera.