To use peptides as synthetic vaccines, they have to be coupled to a carrier protein to make them more immunogenic. Coupling efficiency between a carrier protein and a peptide, however, is difficult to control with respect to loading density of the peptide, This makes these carrier proteins poorly suitable for practicle use. Attempts have been reported to find carrier molecules or delivery systems which allow easy coupling or incorporation of peptides, reproducible loading densities and well defined products. We compared several promising constructs or delivery systems by immunization of male pigs using a tandem GnRH peptide as a branched polylysine construct, a lipo-thioester, a lipo-amide or a KLH conjugate in CFA, and the lipoamide peptide in an immuno-stimulating complex (ISCOM). We found the lipo-thioester and the branched polylysine constructs to be the most effective carrier molecules for the induction of antibodies against GnRH and immunocastration of pigs.