A novel DNA-binding protein complex, the HB complex, has been characterized by means of autoantibodies. Three proteins of 9000, 7500, and 7000 Da constitute the HB complex. The 9000- and 7500-Da proteins are phosphorylated. Autoantibodies recognize the 7000-Da protein when it is bound to DNA. No reactivity against any protein was observed when the complex was dissociated from DNA. The three proteins are acidic (pI 5-6.2), and the complex was able to bind to synthetic double-stranded DNAs of different composition. © 1994 Academic Press, Inc.