The binding of nuclear proteins of hematopoietic cells to transcriptional enhancers of interferon-inducible genes has been studied before and after exposure to alpha-interferon. Mobility shift assays show that a complex formed with interferon-inducible transcriptional enhancers before interferon induction contains a 73- and 84-kDa protein. Amino acid sequencing of the oligoaffinity column purified 73- and 84-kDa proteins showed that they belonged to a family of DNA-binding proteins which have been previously identified to exhibit binding in a sequence nonspecific manner to the ends of fragmented DNA or the origin of replication of adenovirus Type 2 DNA and sequence-specific binding to the distal regions of the U1 small nuclear RNA promoter, the promoter of the transferrin receptor gene, and the transcriptional regulatory regions of HLA genes. Following exposure to alpha-interferon, more slowly migrating complexes appeared which contained a 48-kDa protein, a 95-kDa protein, and a 105-kDa protein which bound to the 9-27 transcriptional enhancer in a sequence-specific manner.