The serum and intracellular stability of 2',3'-dideoxycytidine (ddC) encapsulated in liposomes having different physicochemical properties have been investigated. Results showed that the presence of cholesterol in the lipid composition of liposomes resulted in an increased leakage of ddC when incubated in 80% serum at 37 degrees C. The length of the hydrocarbon chains of the phosphatidylcholine component in cholesterol-containing liposomes did not induce major modifications in both the efficiency of encapsulation and retention of the antiviral agent. The uptake and intracellular stability of the different liposomal formulations have also been evaluated as a function of drug concentration in RAW 264.7 macrophages. For all liposomal formulations tested, an enhanced uptake of liposome-encapsulated ddC by macrophages was observed when the liposomal drug concentration was increased. In addition, the anionic character of liposomes seemed to be an important factor to obtain a high intracellular uptake of ddC. The drug release from liposomal ddC-loaded macrophages has also been evaluated in serum-free medium. Liposomes having long saturated fatty acyl phospholipids and containing 50% (molar ratio) of cholesterol displayed the best stability in the intramacrophagic compartments at all liposomal ddC concentrations used. On the other hand, although the leakage of ddC from liposomes sterically stabilized with polyethyleneglycol chains was similar to that of other cholesterol-containing liposomes, the antiviral agent was readily released from cells for all concentrations of liposomal ddC tested. In conclusion, these results show that the serum stability does not necessarily reflect the intracellular stability, and suggest that some lipid components such as cholesterol can modulate the liposomal stability of drugs such as ddC in response to the conditions of the environment, the properties of the drug used and the nature of interactions between liposomes and cells.