The change in particle size of the liver scanning tin colloid radiopharmaceutical (Amerscan) has been determined using the non-perturbing method of photon correlation spectroscopy. Although samples of the tin colloid taken from the preparation vial immediately after preparation and allowed to remain undisturbed in the light scattering cell grow to sizes of ca. 1,200 nm radius, when samples are taken by syringe from the preparation vial at varying times, the particle size is only ca. 400 nm radius. This effect has been shown to be due to shear deflocculation within the syringe needle. Thus, as the tin colloid is always administered in the clinical situation with a syringe, the size of particle which the patient receives is satisfactory for liver uptake. The tin colloid has been shown to be essentially a charge-stabilised colloidal system from measurements of both zeta potential and effect of electrolyte on the flocculation process.