Abstract: The long migratory flights yield to stressful physiological conditions for birds. We have addressed the study of the immune response of splenic and thymic mononuclear leukocytes from black-headed male gulls Larus ridibundus, searching for a putative effect of the migration cycle on the function of these leukocytes. The immune response was studied after the migratory flight (post-migratory period) and before they start the flight (premigratory period) preceded for a long resting period. The functions assayed were: chemotaxis, lymphoproliferative response to mitogens and natural killer (NK)-like cell-mediated cytotoxicity. Experimental groups were constituted on the basis of the age of the animals, because this parameter is considered to induce different patterns of immune response. We have found that the immune response of the mononuclear leukocytes is affected by the migratory cycle. An immunosuppression was observed in the post-migratory period in chemotactic mobility and even in the response to Con A and LPS of splenic mononuclear leukocytes. Different patterns were observed in the other responses studied. Our results suggest the existence of different sensitivity of the leukocyte subpopulations to the effect of the migratory flight in gulls. Abbreviations: Con A, concanavalin A; cpm, counts per min; FBS, fetal bovine serum; FMLP, N-formyl-Met-Leu-Phe; NK, natural killer; LPS, lipopolysaccharide; PBS, phosphate-buffered saline solution; PHA, phytohaemagglutinin; PWM, pokeweed mitogen.