Blood circulation is the main distribution route for systemic delivery and the possibility to manipulate red blood cells by attaching nanoparticles to their surface provides a great opportunity for cargo delivery into tissues. Nanocarriers attached to red blood cells can be delivered to specific organs by orders of magnitude faster than if diluted in the blood. Another ad-vantage is a shielding from recognition of the immune system, thereby increasing the efficiency of delivery. We present a high-throughput microfluidic method that can monitor the shape of drifting cells due to interactions with nanoparticles and characterize 3D dispersion of fluorescent silica nanoparticles at the surface of red blood cells. The combination of a fluores-cence microscopy technique with image analysis demonstrates that silica nanoparticles adsorption to the surface of red blood cells is strongly influenced by electrostatic interactions. Reduced numbers of intact red blood cells with increasing nanoparti-cle concentration beyond a threshold indicates that there is a toxicity mechanism associated with nanoparticle adsorption at the surface of red blood cells.