In field and pot experiments the effect of selected pesticides on the nitrogenase activity, num­ber of soil microorganisms, and yield of hybrid lucerne was investigated. The obtained results show that crop protection preparations applied in the experiment (seed dressing compound Funaben T and Pivot 100SL herbicide) reduced the activity of nitrogenase in the active strain of Sinorhizobium me­liloti both under conditions of pot and field experiments. In addition, the authors observed a noxious influence of the applied pesticides on nodulation, root development and yield of lucerne. Furthermore, in the first days after their application, the employed herbicide and fungicide inhibited mul­ti­pli­cation of soil microorganisms under lucerne plantations, while later they were found to stimulate their mul­ti­pli­cation.