Protides of the Biological Fluids: Twentieth Volume is a collection of studies about three selected topics from the general field of protein chemistry. The book is divided into four sections. Section A covers monoclonal immunoglobulins and its properties and structure; the amyloid substance; and antibody function. Section B covers active polypeptides derived from plasma proteins; the kinin system; the angiotensin and renin system; anaphylatoxins; and protease inhibitors. Section C covers microcalorimetry in biological systems; biological system and its relation with thermodynamics; and the application of microcalorimetry to biological systems. The book also includes a Section D, which does not consider any of the selected topics, but instead focuses on other techniques such as the double line method; radioimmunoassay of alpha-fetoprotein; and the IgM latex test. The text is recommended for students and reseearchers in the fields of biology, biochemistry, and other related fields, especially those who wish to learn more about monoclonal immunoglobulins, fibrinopeptides, and the effect of microcalorimetry in biological systems.