Protein composition of mesoglea of the scyphomedusa Aurelia aurita was revealed in SDS-PAGE. Some major bands are visible in mesoglea of a mature medusa: 30, 45-47, 85 kDa, three bands between 100-200 kDa, and several bands with molecular weights > 300 kDa. Polyclonal antisera RA45/47 against protein 45 kDa were raised. RA45/47 react with 45-47 kDa protein in mesogleal sample and protein 120 kDa in mesogleal cells on immunoblot. Immunohistochemical analysis of A. aurita histological sections of young and mature medusae showed antigen localization in mesogleal cell granules and in the apical part of ectodermal cells. In mature medusae, the antigen was localized also in elastic fibers. We can conclude that in A. aurita mesogleal cells, along with ectodermal cells, take part in the formation of extracellular matrix of mesoglea.