Biological Properties is a collection of papers that deals with the biological properties of iron-sulfur proteins. One paper reviews the role of electron paramagnetic resonance in forwarding knowledge about iron-sulfur proteins. Iron-sulfur proteins are iron proteins where sulfur is a ligand of the iron, of which the iron is not simultaneously held by a stronger ligand such as porphyrin. Another paper discusses the role of bacterial ferredoxins in coupled oxidation-reduction reactions, the role of hydrogenase in oxidation-reduction, as well as the bacterial iron-sulfur proteins, such as azoferredoxin and molybdoferredoxin. Abiological models of nitrogenase involve molybdenum and iron with sulfur ligands; these abiological systems can be the first models that show the biological activity of iron-sulfur enzymes which these systems try to copy. One paper investigates the role of iron-sulfur proteins in photosynthesis, particularly the photoreduction of ferrodoxin, the mechanism of NADPH formation, and the possible role of ferrodoxin in cyclic photophosphorylation. This collection is suitable for bio-chemists, cellular biologists, micro-biologists, and scientists involved in research in the biological disciplines.