Recent observations on isolated mesogloea (extracellular matrix) of different species of Hydrozoa have demonstrated that cell adhesion and spreading of isolated cells and tissues on this matrix is at least family-specific. The resolution of this technique, however, may be sufficient to help define a species. This was successfully demonstrated with 4 species of Eudendrium: E. capillare, E. ramosum, E. racemosum and a yet undescribed species of Eudendrium. When tissue fragments of polyps were grafted on polyp mesogloea of all 4 species, they only adhered and spread on their own kind, except E. capillare and E. ramosum which demonstrated mutual adhesion and spreading. This method, which uses live material and can be performed in difficult field conditions, could help taxonomist to delimit species.