Chemically synthesized polypeptides containing about 6 to about 40 amino acid residues having amino acid residue sequences that substantially correspond to the amino acid residue sequences of antigenic determinants of interleukin-2, when administered alone or as polymers or as conjugates bound to carriers, induce the production of antibodies of predetermined specificities. The polypeptides and the antibodies produced thereto can be used in diagnostic systems to measure the presence and amount of interleukin-2 and interleukin-2 cell surface receptors or binding sites in an assayed sample.