An obvious difference between the immune system of fish and mammals is that fish lack both bone marrow and lymph nodes; in their place, the head-kidney acts as a major haematopoietic and lymphoid organ in adult fish, whereas the thymus, spleen and mucosa-associated lymphoid tissues are common to both fish and mammals. This suggests that differences exist in antigen presentation and naive lymphocyte stimulation, a prerequisite for the initiation of adaptive immune responses. Intraperitoneal injection of the bony fish gilthead seabream (Sparus aurata L.) with intact Vibrio anguillarum, as a particulate bacterial antigen, results in the mobilisation of head-kidney leucocytes to the peritoneal cavity and priming of their respiratory burst activity. We have also observed the rapid infiltration of acidophilic granulocytes, which are leucocytes functionally equivalent to mammalian neutrophils, into the spleen. These cells may be involved in antigen capture and transport to the spleen, since an apparent association between mobilised acidophilic granulocytes, bacterial antigens and proliferating lymphocytes has been seen in this organ. Collectively, these results suggest that, in addition to being actively involved in bacterial clearance, fish phagocytic granulocytes play a role in the initiation and support of the adaptive immune response.