The in vitro cytotoxic reaction of leucocytes from the seawater teleosts gilthead seabream (Sparus aurata) and sea bass (Dicentrarchus labrax) against tumor target cells was studied by transmission and scanning electron microscopy. Head-kidney, blood or peritoneal exudate leucocytes were incubated with HeLa or B16 melanoma cells. After incubation, conjugates of leucocytes (effectors) and tumor cells (targets) were observed. Both the effector-target cell binding and target cell lysis steps of the cytotoxic process were characterized. Usually more than one effector was bound to the same target. The effectors exhibited ultrastructural features of either monocyte or lymphocyte cells. Monocyte-like effectors possessed an oval or kidney-shaped heterochromatinic nucleus and a few granules. They were flattened against the targets over a broad area or, occasionally, made several punctuated contacts. Lymphocyte-like effectors had a large, rounded or indented nucleus, numerous free ribosomes and occasional cytoplasmic granules. These effectors established spot contacts with the target. Contacts between monocyte-like and lymphocyte-like effectors were also recognizable. After incubation, some targets appeared virtually intact, while others had a smooth surface or showed cell processes and surface blebs. These ultrastructural changes in the targets are similar to those described as mediated by mammalian cytotoxic cells.