Reliable quantities of blood can be sampled repeatedly from the lesser octopus Eledone cirrhosa (Lam. ) and the haemocytes cultured for up to 72 h. Sampling causes an increase in the number of haemocytes/ml, in the percentage of haemocytes containing cytoplasmic granules and a change in the staining pattern of the haemocytes. Sampling also causes a decrease in the amount of copper (mg/ml) in the haemolymph and an initial decrease in the amount of protein (mg/ml), which returns to the original values over extended sampling periods. The haemocytes from E. cirrhosa will phagocytose bacteria (Vibrio anguillarum) in vitro in the absence of haemolymph (dependent on the temperature and duration of incubation) but enhanced phagocytosis will occur in the presence of haemolymph (10- 100% concentration). Opsonization is also dependent on the temperature and on the duration of exposure of the bacterium to the haemolymph. Haemocytes migrate towards low concentrations of blood preparations, to lipopolysaccharide (LPS) and to preparations which had contained live bacteria. Haemocytes also have a bacteriostatic effect on the growth of live bacteria with the effect being dependent on the temperature, duration of the assay and the bacterial species used. Haemocytes also produce intracellular reactive oxygen species, detected by nitroblue tetrazolium, after incubation with dead bacteria in particular, but also live bacteria and LPS. E. cirrhosa haemolymph agglutinates the bacteria V. anguillarum, Y. parahaemolyticus and Aeromonas salmonicida and exerts a bacteriostatic effect on these bacteria which is dependent on the temperature, the amount of haemolymph present and the bacterial species used. The haemolymph, haemocytes and certain tissues from E. cirrhosa exhibit lysozyme and antiprotease activity. The injection of live V. anguillarum causes an increase in lysozyme activity in the branchial heart (after 48 h) and a decrease in the haemocytes (after 24 h). Antiprotease activity increased in the haemocytes (4 h) after bacterial injection but decreased in the haemolymph. Live bacteria caused an increase in the number of circulating haemocytes. The bacteria were cleared from the circulation of E. cirrhosa in about 4h by both the haemocytes and tissues (branchial heart, branchial heart appendage and white body) where they were degraded. The large vacuole in branchial heart cells changed in appearance 4h after bacterial injection and the haemocyte nucleus became pleomorphic. Colloidal graphite was aggregrated in blood vessels only.