Histamine has a key role in the regulation of inflammatory and innate immune responses in vertebrates. Gilthead seabream (Sparus aurata L.), a marine hermaphrodite teleost of great commercial value, was the first fish species shown to possess histamine-containing mast cells (MCs) at mucosal tissues. MCs are highly abundant in the peritoneal exudate of gilthead seabream and compound 48/80 (Co 48/80), often used to promote MC activation and histamine release, is able to promote histamine release from gilthead seabream MCs in vitro and in vivo. The aim of the present study was to analyze the effect of histamine and Co 48/80 on the immune responses of gilthead seabream. For this purpose, histamine and Co 48/80 were intraperitoneally injected alone or combined with 10(9) heat-killed Vibrio anguillarum cells and their effects on head kidney and peritoneal exudate were analyzed. The results indicated that although histamine and Co 48/80 were both able to alter the percentage of peritoneal exudate and head kidney immune cell types, only Co 48/80 increased reactive oxygen species production by peritoneal leukocytes. In addition, histamine, but not Co 48/80, was able to slightly impair the humoral adaptive immune response, i.e. production of specific IgM to V. anguillarum. Notably, both histamine and Co 48/80 reduced the expression of the gene encoding histamine receptor H2 in peritoneal exudate leukocytes. These results show for the first time in fish that although systemic administration of histamine and Co 48/80 is safe, neither compound can be regarded as an efficient adjuvant for gilthead seabream vaccination.