Cells of the reticuloendothelial system (RES e.g. macrophages) play an important role in the immunopathogenesis of AIDS. The objective of the present study was to investigate the possibility of specifically targeting antiviral drugs such as azidothymidine (AZT) to macrophages using nanoparticles as colloidal drug carriers. In a first series of experiments the body distribution of 14C-labelled AZT bound to nanoparticles and a similarly prepared control solution with unbound AZT were studied in rats after intravenous injection. In a second series of experiments polysorbate 80-coated nanoparticles and a solution of AZT in saline were tested. 14C-labelled AZT was bound to nanoparticles using the surfactant bis(2-ethylhexyl) sulphosuccinate sodium (DOSS). The radioactivity in several organs, including those containing large numbers of macrophages, was measured after intravenous injection of the AZT-nanoparticles and the AZT-control solutions. AZT concentrations were up to 18 times higher in organs belonging to the RES if the drug was bound to nanoparticles compared with unbound AZT. These results demonstrate that nanoparticles are a potential drug targeting system for anti-AIDS drugs. The increase in drug concentration at the sites containing abundant macrophages may allow a reduction in dosage to reduce systemic toxicity.