Abstract. Antigen localization studies in the mirror carp Cyprinus carpio were carried out using a soluble antigen, human gamma globulin and a cellular bacterial antigen, Aeromonas salmonicida. Antigen was detected using direct single layer and indirect double layer immunofluorescent techniques, respectively. A study of the spleen, pronephros, mesonephros, liver and thymus at various times after administration of the antigen showed that fluorescence occurs mainly in the ellipsoids of the spleen and in the haemopoietic parenchyma of the pronephros and mesonephros and reaches its greatest intensity when antibody titres are relatively high. A direct comparison with antigen localization in the clawed toad Xenopus laevis, has enabled us to analyse possible evolutionary trends towards the complex dendritic retention of antigen seen in the higher vertebrates.