Three cases of adult linear IgA bullous dermatosis were examined to determine the types of IgA present in the basement membrane zone. IgA 1 subclass, without IgA2, was identified in all three cases; J chain was identified in only one case. Secretory component was absent in all cases. Two observations can be made from this study. First, the predominance of monomeric IgA1 is more compatible with the pattern of antibody secretion of plasma cells present in extragut sites than in intestinal sites. These findings are further evidence of the distinction between dermatitis herpetiformis, which has polymeric IgA1, and adult linear IgA bullous dermatosis and may suggest an extragut site for the origin of the antibodies. The second observation is that the antibodies in adult linear IgA bullous dermatosis show limited expression of heavy and light chains and molecular size that cannot be explained by origin in any compartment. The origin of this limitation cannot be determined from the present data, but possible explanations include a monoclonal or oligoclonal origin of the plasma cells secreting the anti-basement membrane antibodies, genetic restriction of either the immunoglobulin repertoire or helper T-cell response such that only an IgA subpopulation is permitted to be produced in response to the antigen, and selective absorption from the sera or deposition in the skin.