The method of determining the distribution and condition of vesicles within a mammal, said vesicles comprising a lipid bilayer, an ionophore being incorporated in said lipid bilayer, a chelating agent entrapped within the vesicles, and an effective amount of physiologically compatible radioactive tracer bound to said chelating agent within the vesicles, said method comprising:
- (a) determining the initial rotational correlation time of the radioactive tracer through measurement of the time-integrated perturbation factor of said vesicles,
- (b) injecting said vesicles into said mammal,
- (c) observing by scintillation techniques the distribution of said vesicles, and
- (d) determining any change in said time-integrated perturbation factor of the whole body or of parts by appropriate shielding.