Selenocysteine is cotranslationally inserted into proteins by recoding the stop codon UGA from termination to selenocysteine insertion. The nucleophilic selenol group of selenocysteine endows this rare amino acid with unique chemical reactivity that allows regiospecific covalent conjugation in the presence of the other natural amino acids. Using a mammalian expression system, we generated an IgG1-derived Fc fragment with a C-terminal selenocysteine in yields comparable to conventional monoclonal antibodies and conjugated it to an electrophilic derivative of a peptidomimetic that binds with high affinity and specificity to integrin alpha(4)beta(1). Through this conjugation, both the biological and chemical components are endowed with pharmacological advantages. We demonstrate that whereas the Fc protein increases the circulatory half-life from minutes to days and mediates transcytosis through binding to the neonatal Fc receptor, the peptidomimetic introduces cross-species binding to cell surface integrin alpha(4)beta(1) and blocks its interaction with vascular cell adhesion molecule-1. Compared with conventional monoclonal antibodies, our technology benefits economically from combining a generic biological component with a variable chemical component.