The growth in vitro of the endophyte of Alnus glulinosa is studied by determining the increase of the infective capacity of inoculated nutrient solutions during incubation. Growth is only possible if an alcoholic extract from Alnus roots is added to the nutrient solutions. This extract may also contain inhibiting substances but these substances are removed by transfer to petroleum ether. The growth-promoting substance belongs to the non-acid, non-saponifiable lipids. The substance is not adsorbed to ion-exchange resins but is adsorbed from the petroleum ether solution by aluminiumoxyde or silicagel. The growth of the endophyte is inhibited by substances formed during autoclaving of glucose and by yeast autolysate. Peptone is indispensable and can not be replaced by asparagin. Some indications were obtained that growth is stimulated by a water-soluble factor present in Alnus nodules. Even under favourable conditions the growth rate is very low, maximum growth being obtained after 2-3 weeks. In an infective culture, clusters of very thin hyphae were microscopically visible. The results are discussed with regard to the conflicting results obtained by Pommer and a working-hypothesis is suggested to reconcile the different observations.