THE position of hydroxylamine as an intermediate in the nitrogen fixation process has become increasingly doubtful as a result of recent studies1,2. No direct attempts have been made, however, to find the effect of hydroxylamine on nitrogen fixation. An application of the isotope tracer method to this problem is being made in these laboratories, using nitrogen containing 30 per cent nitrogen-15. A large 18-hour-old culture of Azotobacter vinelandii in a nitrogen-free mannitol medium was aseptically subdivided into equal 100-ml. portions, and 1 ml. of suitable dilutions of buffered hydroxylamine added to give the concentrations required for inhibition measurements. The cultures were then aerated in parallel with an atmosphere containing nitrogen (partial pressure, 0·25 atm.), and oxygen (partial pressure, 0·2 atm.)—the nitrogen being 'heavy'—in a thermostat at 29° C. After a four-hour period of aeration, the cells were collected, and nitrogen samples prepared for isotope analysis. The following analyses were obtained :