The aim of this paper is to investigate the possibility of reducing mineral nitrogen fertilizers with the application of nitrogen-fixing bacteria, using modern technological methods. In agricultural production, nitrogen is an important element for the growth and development of plants and, thus it helps achieve optimal yields. The largest reserves of this element can be found in the atmosphere, where the nitrogen exists in its molecular form, which plants are not able to take and utilize. Biological nitrogen fixation is a process in which specific groups of micro-organisms (nitrogen-fixing bacteria) take elemental nitrogen from the atmosphere and with the nitrogenase enzyme reduce it to ammonia, which becomes available to plants for their metabolism. According to the relation of microorganisms and higher plants, nitrogen fixation can be free, associative and symbiotic. At free nitrogen fixation microorganisms are not directly, histologically connected to higher plants, but they are free in soil or water, while the associative nitrogen fixation takes place with the help of micro-organisms living on the surface of the root or in the root of the plant. The most important type of symbiotic nitrogen fixation for agricultural soils is the nitrogen fixation that occurs as a result of the symbiosis between the bacteria of the genera Rhizobuim, Bradyrhizobuim, Sinorhizobium and the plants, mostly of the family Leguminosae. The free nitrogen fixation in the soil mainly occurs as a result of saprophytic bacteria of the genus Azotobacter and Clostriduim, but also several other genera of free-living bacteria and cyanobacteria, while the most important associative nitrogen-fixing bacteria are those of the genus Azospirillum. Biological nitrogen fixation ensures a sufficient amount of nitrogen nutrients for the plants, and fully meets the requirements of land management, such as productivity, safety, protection of natural resources and economy. With the increased use of biopreparations containing effective strains of nitrogen-fixing bacteria, as well as with the development of the top-quality varieties of leguminosae it can be expected that in the future we will reduce the use of chemical fertilizers and pesticides and that we will increase the efficiency of agricultural production.