This paper reviews our studies on a 24-residue synthetic peptide representing part of the amino acid sequence of the influenza virus hemagglutinin. We have used this peptide as a model antigen to define short sequences and individual amino acid residues involved in and critical for interaction with antibody and with T cells. These studies provide insight into the way in which an immunogen is viewed by the immune system and also the minimum requirements necessary for the expression of immunogenic and antigenic activity. This information is helping us in exploiting synthetic peptides in the construction of designer immunogens which have potential as candidate vaccines.