It is accepted that some serum components play important roles in enhancing liposome permeability and in facilitating rapid liposome uptake by the mononuclear phagocytic system. In this study we systematically investigated the influence of serum components from different species on complement-mediated immune damage to hydrogenated phosphatidylcholine (HEPC)-based liposomes. Our results demonstrated that when liposomes were incubated with fresh serum from rats or bovines, there was obvious leakage of 5(6)-carboxyfluorescein (CF) from the liposome. However, when liposomes were incubated with fresh serum from humans, rabbits, guinea pigs, mice, and dogs, almost no pronounced leakage from the liposome was observed. These results indicate that the variability of damage to a liposome corresponds to the variability of the animal species from which the serum comes. In addition, leakage of CF from liposomes was completely inhibited by heating at 56 degrees C for 30 min or by treatment with EDTA. However, such leakage was not blocked by treatment with EGTA/Mg2+, suggesting that the mechanism of lysis of liposomes is due to complement activation via the alternative pathway rather than via the classical pathway. Studies on reconstitution and compatibility further confirm that some serum factors (complement activating factors, CAFs) induce the activation of the complement system, ultimately leading to the lysis of the liposomes. However, CAF from different animal species exhibited corresponding species differences. Meanwhile, under the condition of heating and dialysis experiments, it is obvious that the CAF is susceptible to heat and the dialysed serum sustains biological activity to destabilize liposome following dialysis against a buffer with Ca2+ and Mg2+, indicating that the CAF is not a type of low-molecular weight material but a serum protein.