Functional-lipid constructs of the structure F-S-L are disclosed, wherein F comprises a tyrosine or histidine residue, S is a spacer covalently linking F to L and L is a lipid. The functional residue may be iodinated and used to radiolabel a biological entity with 1251 and the iodinated constructs are used in a non-invasive method of monitoring the distribution of a biological entity in vivo.