Bovine serum albumin (BSA) and anti-bovine serum albumin (a-BSA) were separately incorporated in dioctadecyldimethylammonium bromide (DODAB) liposomes in water or in phosphate buffer saline (PBS). In water, 100% BSA or a-BSA incorporation in the liposomes contrasts with ca. 30% incorporation in PBS for both proteins, suggesting a major role for the electrostatic attraction between protein and liposome in determining the incorporation, though 30% incorporation can still be ascribed to the hydrophobic interaction. Neither of the two proteins, tested separately, can induce nonspecific liposome aggregation over an extensive range of combinations for DODAB and protein concentration as those possible in a microplate experiment. In water, electrophoretic mobility for the liposomes decreases slightly as a function of protein concentration but the liposome/protein complex remains positively charged even at the highest protein concentrations tested. Protein-induced rupture of liposomes containing [C-14]sucrose was evaluated from dialysis of protein/liposome mixtures. In water or in PBS, protein-induced leakage of radioactive liposomal contents was not observed, suggesting that the hydrophobic interaction between serum proteins and the DODAB bilayer is superficial. Finally, the absence of molecular recognition between BSA and a-BSA separately incorporated in the liposomes in water is consistent with preservation of the liposomal positive charge at full protein coverage on the cationic liposomes. These results shed new light onto the mechanisms by which cationic liposomes can deliver proteins or other important macromolecules of biological origin in vivo : opsonization possibly preserves liposome charge and integrity.