The haemolytic and peroxidative effects of crude venom of the sea anemone Stichodactyla helianthus were evaluated in rat and human erythrocytes. Venom extract caused a significant concentration-dependent effect on haemolysis (release of haemoglobin). Human erythrocytes were more sensitive (0.094 mg protein/ml) than those of the rats (0.3787 mg protein/ml). In contrast, a light effect on lipid peroxidation (LP, an index of oxidative damage to membrane lipids) was recorded. The concentrations needed to produce a significant effect on LP in rat and human erythrocytes were, respectively, 2-fold and 7-fold higher than those required to produce significant haemolysis. The differential effect of S. helianthus venom on haemolysis and oxidation of membrane lipids is not common for venoms of other sea anemones, which usually show a tightly related effect on LP and haemolytic damage.