THE exact requirements of the nitrifying bacteria for inorganic nutrients are not yet known, though all workers on this group since Winogradsky1 have assumed that they require iron, and many workers have added traces of other metals to their media2, or used tap-water as a diluent, because of the possible trace elements it contains3. One of us has recently discovered4, by the percolation method5, that copper is apparently essential for nitrification in soil, so we decided to try the effect of iron and copper, alone and with other trace elements, on nitrite formation in cultures of nitrifying bacteria.