Molybdenum is unique among the elements of the second and third transition series of the Periodic Table in having an essential role in several biochemical reactionsl. The most important of these concern the utilisation of inorganic nitrogen for the production of proteins, nucleic acids and other nitrogenous cell constituents2–4. Living organisms differ widely in their ability to synthesise the amino-acid precursors of these molecules and with respect to the forms of nitrogen they can utilise for this purpose. Higher animals are unable to synthesise certain amino acids-the essential amino acids-and must obtain them from an exogenous source. Furthermore, they cannot utilise the most abundant natural sources of inorganic nitrogen, namely soil nitrate and atmospheric nitrogen, in the synthesis of even the nonessential amino acids. Fortunately, plants and many micro-organisms are more versatile; they can make all the amino acids found in proteins starting from either nitrate or ammonia. The metabolic assimilation of nitrate into the form of ammonia, proceeds in two major steps: (i) reduction of nitrate to nitrite and (ii) reduction of nitrite to ammonia. The first of these reactions is catalysed by the molybdenum-containing enzyme, nitrate reductase. Ammonia is also produced by the bacterial fixation of molecular nitrogen. This is accomplished by free-living soil bacteria, for example Clostridium pasteuranium and, more importantly, by degenerate forms of other soil bacteria, for example Rhizobium in association with root nodules of leguminous plants. Both fixation processes are catalysed by another molybdenum-containing enzyme, nitrogenase. Thus molybdenum has a central role in the principal routes of nitrogen incorporation into plants and therefore animals. It has been estimated that approximately 1010 tons of nitrogen are incorporated annually into plants5. In this respect alone, molybdenum must be considered one of the most important of the biologically active metals.