1. It is impossible to cultivate the endophyte from the root-nodules of the alder on the usual media in vitro. 2. Methods are described by which one can demonstrate with certainty whether certain organisms are capable of forming root-nodules in the alder. 3. The difficulties in connection with isolation are not caused by a dying off or removal of the endophyte during the required pre-treatments (disinfection with sublimate or bromium, removal of outer cell layers, suspension in water or nutrient media). 4. The endophyte is rather sensitive with regard to the influence of higher temperatures. During 1 hour at 60° C. the capacity to infect is completely lost. 5. Nodules which have remained visibly sterile after remaining for 6 weeks on a good nutrient medium, contain the endophyte still in its vital, virulent and efficient form. 6. Such nodules are very suitable as inoculation matter for work under sterile conditions. 7. With the aid of such nodules as inoculation matter elementary nitrogen fixation could be confirmed under pure conditions.