The work deals with the description of a novel antigen marker of differentiated foot cells of polyps of the Hydra genus. The antigen called 3G11 has been revealed by a monoclonal antibody of the same name in granules of the ectodermal secretory cells of the basal disc. The appearance of the antigen in the developing foot tissue during budding and regeneration, as well as in the ectopic foot tissue, indicates the differentiation of epithelial cells of the gastral part into secretory cells of the basal disc. The 3G11 antigen is species specific; of the six studied species, it was only revealed in hydra of the taxonomic group vulgaris, a supraspecies taxon of the Hydra genus. The 3G11 antigenic marker and the previously described biochemical marker, peroxidase of the basal disc, have been shown to have similar localizations in the polyp tissues. However, the data form the immunoenzyme analysis indicate that the molecule carrying the 3G11 marker does not have peroxidase activity. Thus, the novel 3G11 antigenic marker supplements the series of the previously described differentiation markers and allows for the procedure for detecting differentiated tissue of the hydra basal disc to be improved.