Cellular entry is important for the delivery of potential drugs, small-molecule modulators and sensors. Recently attention has focused on the development of a diverse range of specific delivery systems in order to enhance transport and uptake at the cellular level. Contemporary examples of delivery vectors include peptides, such as those based on fragments derived from the Tat protein, peptidomimetic carriers, such as those based on polymers and dendrimers, and various physical methods, such as micro-injection. [1][2][3][4][5][6][7] Polymer-mediated delivery offers many advantages; for instance, polymeric materials can allow the controlled delivery and release of their cargo at controllable local concentrations, and there are many examples of drugs and sensors being encapsulated within polymer microparticles. However, their introduction into cells is often complex.[7] One type of polymeric material that has been shown to be engulfed and actively transported throughout cells is latex beads; however, surprisingly little work has been carried out on exploiting this phenomenon.[8] One reason for this is that commercially available materials are often unfunctionalized chemically or, if they are functionalized, this is only achievable under mild aqueous conditions, whereas modification for the attachment of a range of more interesting chemical probes requires compatibility with organic solvents.The aim of this study was the generation of robust biocompatible polymeric particles of defined sizes that could be employed in solid-phase multistep syntheses yet be taken up rapidly by cells and exploited in a variety of applications. Here the application of a range of functionalized, cross-linked, multilabelled polymer microspheres as a mode of traversing the membrane of living cells is described. It allows general cellular labelling, sorting and the potential monitoring of dynamic intracellular processes.Uniform, monodisperse amino-functionalized microspheres (0.2 and 0.5 mm) with 2 % divinylbenzene (DVB) cross-linking were prepared by an emulsifier-free emulsion polymerization, while 2.0 mm microspheres were synthesized by dispersion polymerization (Scheme 1). [9,10] By selecting the relevant polymerization conditions, it was possible to generate functionalized microspheres of any desired size and loading (0.02-0.20 mmol g À1 ). As expected, the cross-linking component was found to be vital to enhance the robust nature of the microspheres during routine chemical manipulations.The microspheres were labelled with fluorescein, rhodamine, Texas Red or dansyl dyes as necessary (synthesis details are given in the Supporting Information), and a number of cell studies were conducted. Mouse melanoma (B16F10), HeLa, human embryonic kidney (HEK-293T) and neuronal ND7 cells were treated in triplicate with the polymeric microspheres (0.2-2.0 mm) at a range of concentrations (0.2-0.8 mg mL