The organ-specific, phylogenetic, and ontogenetic distribution of the epidermolysis bullosa acquisita (EBA) antigen, a newly recognized basement membrane component, was determined using polyclonal sera from patients with EBA and a mouse monoclonal antibody, H3a. Both antibodies are distributed at the basement membrane zone of skin, oral, anal, and vaginal mucosae, and esophagus, but not in kidney, urinary bladder, lymph nodes, placenta, or blood vessels. Both polyclonal and monoclonal EBA antibodies react with a basement membrane antigen in primate and other mammalian skin, but not in avian, amphibian, or reptilian skin. The antigen is present initially in the 8-week-old human fetus, and increases in density until the adult linear pattern is reached at 15 weeks' gestational age.