A new antigenic marker of the differentiated basal disk cells of Hydra was characterized. An antigen named 3G11 was revealed by monoclonal antibody in granules of the basal disk gland cells of the ectoderm. The antigen appearance during budding, regeneration and ectopic foot formation evidences for the differentiation of the body column epithelial cells into basal disk gland cells. Antigen 3G1 1 is species-specific: among six hydra species investigated, the antigen was observed exclusively in polyps of vulgaris group which is a special taxon of the filum Hydra. Cell and tissue localization of the antigen 3G11 was similar to that of the well-established biochemical hydra marker, foot specific peroxidase, reported formerly. However, ELISA data suggest that the molecule bearing antigen 3G11 does not possess any peroxidase activity. Thus the new hydra antigenic marker 3G11 extends the number of previously used markers of differentiation and allows to improve the technique of the basal disk differentiated tissue identification.