The haemolysis caused by various types of liposomes was measured after incubation of liposomes with human red blood cell (erythrocyte) suspension. Liposomes composed of phospholipids and containing or not arsonolipids (arsonoliposomes) were tested. In some cases liposomes that were coated with polyethylene glycol (MW 2000), which were formulated by including 8 mol% DSPE-PEG2000 in their lipid membrane, were used. Multilamellar vesicles were prepared by the thin film hydration technique (conventional liposomes) or by the one-step technique (arsonoliposomes). Sonicated vesicles were produced by probe sonication of the initial liposome preparations. Phospholipid concentration in the liposome dispersions were measured by the Stewart assay, and adjusted accordingly. Haemolysis was measured after incubating 100 microl of liposome dispersions with 900 microl of red blood cell suspension (blood) for 1 h. The results reveal that the haemolysis caused, when liposomes are incubated in blood at concentrations below 0.16 mg (lipid)/ml (blood), was minimum. Only in case of Pegylated arsonoliposomes, significant haemolysis percents were observed. At higher lipid concentrations, 0.38 or 0.6 mg/ml, the haemolysis caused by arsonoliposomes was substantially increased, even in the cases of non-Pegylated arsonoliposomes. In most cases, especially when arsonolipid-containing liposomes were evaluated, vesicle size also had considerable effect on vesicle-induced haemolysis. Nevertheless, at concentrations which are relevant with liposomal drug administration in humans, all formulations tested demonstrated negligible haemolysis.