Abstract The utility of an iodinated derivative of inulin ( 125I-tyraminyl-inulin, 125ITI) for reporting in vivo tissue distributions of liposomal contents is described. It is shown, employing a rat model, that this probe satisfies the criteria that the free form is rapidly cleared from the circulation and excreted, whereas 125ITI encapsulated in large unilamellar vesicle (LUV) systems and subsequently taken up in various tissues exhibits a long (>3 days) retention time. Further, high specific activities (>1 μCi per μ1) are easily achievable, allowing low LUV dose levels (≤2.5 μmole phospholipid/kg body weight) to be employed. Minimal tissue workups for quantitation of 125ITI distributions are required. It is concluded that from criteria of sensitivity, expense and simplicity, 125ITI is a most convenient probe for characterizing liposome deposition in vivo .