The role of macrophages in the clearance of particles with diameters <100 nm (ultrafine or nanoparticles) is not well established, although these particles deposit highly efficient in peripheral lungs, where particle phagocytosis by macrophages is the primary clearance mechanism. To investigate the uptake of nanoparticles by lung phagocytes, we analyzed the distribution of titanium-dioxide particles of 20 nm count median diameter in macrophages obtained by bronchoalveolar lavage at 1h and 24h after a one-hour aerosol inhalation. Differential cell counts revealing >96% macrophages and <1% neutrophils and lymphocytes, excluded inflammatory cell responses. Employing energy-filtering transmission electron microscopy (EFTEM) for elemental microanalysis, we examined 1594 macrophage profiles in the 1h-group (n=6) and 1609 in the 24h-group (n=6). We found 4 particles in 3 macrophage profiles at 1h and 47 particles in 27 macrophage profiles at 24h. Model-based data analysis revealed an uptake of 0.06-0.12% ultrafine titanium-dioxide particles by lung-surface macrophages within 24h. Mean (SD) particle diameters were 31 (8) nm at 1h and 34 (10) nm at 24h. Particles were localized adjacent (within 13-83 nm) to the membrane in vesicles with mean (SD) diameters of 592 (375) nm at 1h and 414 (309) nm at 24h, containing other material like surfactant. Additional screening of macrophage profiles by conventional TEM revealed no evidence for agglomerated nanoparticles. These results give evidence for a sporadic and rather unspecific uptake of TiO2-nanoparticles by lung-surface macrophages within 24h after their deposition, and hence for an insufficient role of the key clearance mechanism in peripheral lungs.