The present invention relates to novel single chain interleukin-12 polypeptides. The invention also relates to isolated nucleic acids encoding the single chain interleukin-12 polypeptides, to vectors and cells comprising them, and to their uses, in particular in methods of using single chain IL-12 polypeptides, polynucleotides, vectors and cells of the invention for enhancing immune system function, for example as vaccine adjuvants and in the treatment of infections and cancer.