Immunoglobulins bind antigens and express individual antigenic specificities mainly through residues located in hypervariable loops of their N-terminal domains. Hyper-variable loops are kept in place by a molecular scaffold organized in a sandwich-like structure with two β-sheets stabilized by a disulfide bridge (the immunoglobulin fold). This structural feature, together with the possibility of obtaining high level expression, extracellular secretion, easy purification and stability of the protein product, render immunoglobulin an ideal ‘molecular vehicle’ for the expression of exogenous peptides. Here we report on the engineering of an immunoglobulin expressing an exogenous epitope, the repetitive tetrapeptide Asn-Ala-Asn-Pro (NANP)₃. By recombinant DNA techniques, we inserted three copies of the tetrapeptide (NANP)₃ in the third hypervariable loop (D region) of an immunoglobulin heavy chain variable domain. We show that the engineered antibody was properly assembled and secreted. A panel of polyclonal and monoclonal antibodies, including anti-synthetic peptides and anti-(NANP)_n antibodies, were used to study the molecular configuration of the engineered domain's surface. The results indicate that (i) the exogenous sequence did not appreciably alter the overall fold of the variable domain; and (ii) the inserted epitope folded with a configuration immuno-logically similar to the one assumed in the native protein, suggesting that short- and medium- rather than long-range interactions stabilized the structure of the (NANP)₃ peptide in the folded protein. We propose this system for the expression of peptidic sequences, and their structural and functional analysis.