We describe the design of fluorescent, thermoresponsive microgels surface-functionalized with folic acid. Incubation of these particles with KB cells grown in folate-free medium results in efficient endocytosis of the particles via a receptor-mediated pathway. Laser scanning confocal microscopy and flow cytometry show efficient uptake of folate-modified particles over cationic control particles. Staining of the cells with Lysotracker red, followed by confocal imaging, shows anticorrelation between the particle and endosome fluorescence, which is taken as evidence of particle escape from the endosomes to the cytosol. Finally, the strong dependence of particle swelling on temperature was used to induce particle collapse and aggregation following uptake, which causes significant cytotoxicity. Thus, we have developed polymeric nanoparticles that may display antitumor activity, as they effectively target cancer cells and undergo endosomal escape to the cytosol, and they can then be triggered to cause cell death.