The main functional characteristics of haemocytes from the common periwinkle Littorina littorea(phagocytic ability, acid phosphatase activity, cytotoxic properties and generation of reactive oxygen intermediates)were investigated. The blood cells of L. littorea demonstrated phagocytic activity for zymozanparticles in both plasma and seawater. However, the level of phagocytosis in plasma was higher than inseawater, suggesting the presence of some soluble factors with opsonizing activity for yeast cell walls in thesnail haemolymph. Acid phosphatase was detected in haemocytes following phagocytosis of zymosan.Zymosan particles as well as soluble inducers of respiratory burst (mannan, phorbol-myristate acetate,lipopolysaccharide from Escherichia coli) were shown to trigger superoxide anion production in L. littoreablood cells as evidenced by nitroblue tetrazolium (NBT) reduction. Haemocytes exposed simultaneouslyto both inducer and the superoxide scavenger enzyme—superoxide dismutase demonstrated a lower abilityto reduce nitrobluetetrazolium. Periwinkle blood cells showed plasma-independent cytotoxic activity forhuman erythrocytes which may be due to the release of superoxide intermediates into the extracellularenvironment. These results, together with previously obtained data, suggest that haemocytes are the maineffectors in the internal defence system of L. littorea, with humoral factors playing an accessory role inrecognition and elimination of pathogens.