Liposomes are disclosed which prolong the bioavailability of therapeutic peptides or proteins. The liposomes are unilamellar or oligolamellar vesicles which are stable in lymph, but unstable in serum or plasma. The net electric charge of the lipid vesicle is negative or neutral and the vesicle contains less than 10 mole percent cholesterol. Bioavailability of the therapeutic peptides or proteins is enhanced by combination of the liposomes with an isotonic carrier solution having an osmolarity of between about 200 to 400 mosmol. Bioavailability is further enhanced by subcutaneous injection of the liposomes. The addition of empty liposome to liposome encapsulated therapeutic agents was found to increase bioavailability. Further, the use of empty liposomes as a diluent for solutions of free peptides end proteins increases shelf life.