Polyclonal antibodies, widely used in research and diagnostics, are conventionally isolated from the blood of immunised mammals, especially rabbits. The fact that antibodies can also be detected in the yolk of eggs laid by immunised hens, led to the development of the yolk antibody technology as an alternative method less stressful to animals. Since hens can be kept under nearly natural conditions and antibodies be isolated from the collected eggs, this technology has become an interesting alternative to the ,bloody" techniques. For their work on yolk antibodies, the authors of this book received the FISEA prize (International Foundation for the Substitution of Animal Experimentation, Luxembourg) in 1997. Protocols on how to keep and immunise hens and on the extraction, isolation and use of antibodies from yolk are described in detail in this manual. These practical instructions are complemented by a short introduction to the hen's humoral immune system and a section on the pros and cons of chicken yolk antibodies compared to those of mammals'serum. Avian antibodies have been shown to be in some applications even more effective than mamalian antibodies, especially when phylogenetically highly conserved antigens have been used for immunisation.