The action of different forms of haemoglobin (oxy-, carboxy-and methaemoglobin) and myoglobin on the leakage of Rb+ out of liposomes has been investigated. The results presented will demonstrate that only methaemoglobin is particularly effective in interacting with phospholipid vesicles by changing their permeability and catalyzing a peroxidation of their unsaturated hydrocarbon chains.