The N-compounds excreted from the root nodules of leguminous plants have been isolated and characterized. Over 90% of the excreted N is amino-N. In addition. 1-2% oxine-N and some nitrite-N are found. Most of the amino-N is present as l-aspartic acid, if peas are harvested at a young stage, long before flowering. In ageing cultures the amount of aspartic acid decreases. The other ammo-acid found among the excretion products is [beta]-alanine, which is slowly formed from l-aspartic acid by the legume bacteria. Its formation explains the decrease of aspartic acid in ageing cultures. The oxime appearing among the excretion products is oximinosuccinic acid; nitrite-N is formed from this oxime. Some fumaric acid is detected as an N-free excretion product. The often exceptionally high amount of excreted N (60-80% of the total fixed N) and the special nature of the excreted N-compounds prove that these N-compounds are products of N-fixation. An idea has thus been obtained, on the basis of the excreted N-compounds, of the mechanism of biological N-fixation.