Human monocytes/macrophages (MO/MAC) were isolated from peripheral blood and cultivated on hydrophobic Teflon membranes. This culture system is suitable for HIV infection of MO/MAC in vitro. After transfer into 24-well plates the mature macrophages (infected or uninfected) were used for measurements of phagocytosis. The uptake of different, radioactively labeled nanoparticles (NP) made of polyalkylcyanoacrylate, polymethylmethacrylate (PMMA), and human serum albumin (HSA) by the macrophages was determined. In addition, the influence on phagocytosis of size and composition, concentration, and surface of the NP was studied. Further, macrophages of different state of activation were tested. NP made of polyhexylcyanoacrylate (PHCA) or human serum albumin with a diameter of about 200 nm were found most useful for targeting antiviral substances such as azidotymidine to macrophages. Cells infected in vitro with HIV-1D117/III, a monocytotropic HIV isolate from a perinatally infected child, possessed an even higher phagocytotic activity than noninfected cells. Macrophages isolated from HIV-infected patients also showed good incorporation of NP. Thus, the concept of a specific targeting of antiviral substances to macrophages in HIV-infected individuals appears quite promising.