Synthetic antigens are often valuable tools in immunological research. They are usually linear or branched polypeptides, prepared by polymerization of N-carboxy-α-amino acid anhydrides or by polymerization of defined oligopeptides. Owing to their great variability, these simple antigens allow a systematic study of the chemical and physical parameters involved in immunogenicity, i.e. the ability of a substance to initiate an immune response in higher animals. The study of very simple synthetic antigens made possible the first detailed analysis of genetic factors controlling the immunogenicity of a substance. The results have led to a better understanding of many immunological phenomena, and have helped to explain the complex processes initiated by antigens that lead to a specific immune response.