Legemoglobin (Lb) is a myoglobin-like protein of about 16KDa, which occurs in legume root nodules at very high concentrations. Usually the heme moiety is synthesized by th bacteroids but mitochondria may provide also heme for Lb when bacteria are defective in heme production or perhaps when Lb is produced in unifected cells of nodules. Lb plays and essential role in the nitrogen fixation process, by providing oxygen to the bacteroids at a low, but constant, concentration, which allows for simultaneous bacteroid respiration and nitrogenase activity. Lb must be in the reduced, ferrous state to carry oxygen. Several factors within the nodules are conducive for Lb oxidation to its ferric, inactive form. During these inactivation reactions free radicals are generated. However, healthy nodules contain around 80% of ferrous Lb and 20% of oxyferrous Lb, but not ferric Lb, which indicates that mechanisms exist in the nodules to maintain Lb reduced; these are the enzyme ferric Lb reductase and free flavins Lb degradation is a largely unknown process, but several intermediates with modified hemes, presumably by oxidative attack, have been encountered, including modified Lbam, choleglobin, and biliverdin.The original work reported here has been financed by grants PCB-10/90 (CONAI-DGA), AGR91-0857-C02-02 (CICYT), and PB92-0058 (DGICYT).Peer reviewed