A study has been made of the nature and origin of the organic nitrogenous products to be found in filtrates from cultures of the nitrogen-fixing blue-green alga Anabaena cylindrica Lemm. The total amount of combined nitrogen liberated in extracellular form is relatively greatest in young cultures. At this stage the amounts produced have not been found to be affected appreciably by variation in culture conditions. In the later stages of culture production of extracellular nitrogenous substances appears to be an invariable concomitant of growth, but when comparisons are made at equivalent stages of growth such production is found to be considerably increased by deficiency of nutrient elements such as iron and to be slightly decreased by deficiency of molybdenum in the medium. Production is not dependent to any marked degree upon the volume of the culture medium, nor upon the presence or absence of available combined nitrogen or of glucose in the medium, nor upon variation of light intensity sufficient to affect growth rate to a marked extent. The extracellular combined nitrogen produced by A. cylindrica has been found to consist mainly of polypeptide, together with lesser amounts of amide, the proportion of which decreases as the cultures age, and is of the same general nature when the alga is grown under a number of different conditions. A preliminary examination of the polypeptides has been made employing paper partition chromatography. Neither the peptide nor the amide nitrogen liberated by A. cylindrica is a specific product of the fixation of free nitrogen. The polypeptides appear to be liberated in the course of metabolism at the outer surface of the cell, and the effects of various factors upon their production may thus be accounted for. The ecological effects of these extracellular products of A. cylindrica and similar blue-green algae are discussed, and it is suggested that they may be of particular importance in fresh water because of their capacity to form soluble complexes with various inorganic nutrients.