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 zymozan particles in both plasma and seawater. However, the level of phagocytosis in plasma was higher than in seawater, suggesting the presence of some soluble factors with opsonizing activity for yeast cell walls in the snail 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. littorea blood cells as evidenced by nitroblue tetrazolium (NBT) reduction. Haemocytes exposed simultaneously to both inducer and the superoxide scavenger enzyme—superoxide dismutase demonstrated a lower ability to reduce nitrobluetetrazolium. Periwinkle blood cells showed plasma-independent cytotoxic activity for human erythrocytes which may be due to the release of superoxide intermediates into the extracellular environment. These results, together with previously obtained data, suggest that haemocytes are the main effectors in the internal defence system of L. littorea, with humoral factors playing an accessory role in recognition and elimination of pathogens.