Abstract: The immune response of thymic cells from female blackheaded gulls Larus ridibundus has been studied searching for changes associated to the migratory cycle. The functions assayed were those considered most representative of the cellular immune response: chemotaxis, lymphoproliferative response to PHA and Con A, and Natural Killer (NK)-like cell-mediated cytotoxicity. These functions were assayed in the post-migratory and the premigratory periods, since these migratory periods could produce the highest difference in the response. Experimental groups were constituted on the basis of the age of the animals, because this parameter is considered to induce differences in the immune response. Regarding the effect of the migratory cycle, it was obtained a different pattern of response depending on the function studied. Thus, lymphoproliferation and NK-like cell-mediated cytotoxicity did not show any migration-associated variation. Chemotaxis indexes were lower in the post-migratory period compared to the values obtained in the premigratory period in juvenile gulls. These findings demonstrate that the mobility of thymic cells is depleted after the long migratory flight in juveniles, suggesting that juveniles could be more sensitive to the stressful conditions of the migration. Abbreviations: Con A, concanavalin A; cpm, counts per min; FBS, fetal bovine serum; FMLP, N-formyl-Met-Leu-Phe; NK, natural killer; PBS, phosphate-buffered saline solution; PHA, phytohaemagglutinin.