Conjugated polymer nanopartieles are formed by precipitation of highly fluorescent conjugated polymers to font) small nanoparticles with extremely bright fluorescence. We characterized cellular uptake and cytotoxicity of 18 +/- 5 nm PFBT conjugated polymer nanoparticles in 1774A.1 cells. Significant nanoparticle uptake was observed, indicating efficient nanoparticle entry into cells, even or short (1 h) incubations. The high fluorescence of these nanoparticles allows extremely low loading concentrations; PFBT. nanoparticle fluorescence in cells could be detected with loading concentrations of 155 pM (270 ppb). Cellular uptake slows at low temperature, consistent with endocytic entry. Nanoparticles colocalize with Texas Red dextrin and are trafficked to lysosomes, as demonstrated by the location of nanoparticle fluorescence in pet muck. organelles that also stain with an anti-LAMP-1 antibody. Inhibition of uptake by phosphoinositide 3-kinase inhibitors implicates macropinocytosis as the operative endocytic mechanism. No significant cytotoxic or inflammatory effects could be observed, making PFBT nanoparticles attractive probes for live cell imaging.