Double diffusion in agarose was employed for the characterization of a soluble nucleoprotein antigen that gave precipitin reactions with sera from patients with systemic lupus erythematosus. The soluble antigen that was isolated from calf thymus nuclei was demonstrated by enzyme degradation and ultracentrifugation studies, and by immunologic analysis, to be a complex of deoxyribonucleic acid and a moiety susceptible to proteolytic digestion. Antibody to soluble nucleoprotein did not react with the DNA portion of the complex released after proteolytic digestion or with purified calf thymus DNA. Immunologic reaction of identity was obtained between the soluble nucleoprotein antigen and synthesized DNA-histone complex, suggesting that the protein moiety of soluble nucleoprotein might in part be composed of histone. Antibody to soluble nucleoprotein was present in 51% of systemic lupus erythematosus sera examined and was more common than antibody to deoxyribonucleic acid.