We have studied the properties of a factor or factors that bind selectively to the 5' flanking region of the chicken adult beta-globin (beta A-globin) gene. We previously showed that these components, when bound with histones on plasmids containing the region, confer on the complex a pattern of hypersensitivity to nuclease digestion similar to that in the nucleus. We have now measured the abundance of the factor(s) in partially purified preparations, and the affinity constants for binding to specific and nonspecific DNA sequences. Footprinting studies of the specific complex with DNAase I and II reveal two discrete protected regions within the hypersensitive domain. When these regions are physically separated, they interact with the factor(s) independently, suggesting that each region binds one or more distinct components. The footprint patterns of our complexes in vitro agree with the patterns observed in intact chicken erythrocyte nuclei. These complexes thus are on the transcriptionally active beta A-globin gene in vivo.