This study compares the transport and trafficking of flexible filaments (filomicelles) with spherical particles of similar chemistry in rodents. Filomicelles, which are polymer micelle assemblies, persist in the circulation up to one week after intravenous injection, which is about ten times longer than their spherical counterparts and more persistent than any known synthetic nanoparticle. Under fluid flow conditions, spheres and short filomicelles are taken up by cells more readily than longer filaments because the latter are extended by the flow. Preliminary results show that filomicelles can effectively deliver the anticancer drug paclitaxel and shrink human-derived tumours in mice. These findings suggest that long-circulating vehicles need not be nanospheres and provide insight into possible shape effects of natural filamentous viruses. Filomicelles are cylindrical micelles self-assembled from block copolymers with hydrophilic and hydrophobic chains. They have a hydrophilic chain of polyethylene glycol (PEG) and a hydrophobic chain of either inert polyethylene or biodegradable polycaprolactone. Filomicelles persist in the circulation for up to one week, with longer filomicelles persisting longer. Filomicelles enter cells under static conditions but are opposed by flow. The length of filomicelles affects their ability to enter cells and their fragmentation. Degradable filomicelles of OCL3 show a similar but more sustained decrease in length due to hydrolysis. Filomicelles loaded with paclitaxel shrink tumours, with longer cylinders proving more effective at a given dose. Filomicelles interact with phagocytic cells, with longer filomicelles being less likely to be taken up due to flow-induced extension. Filomicelles can enter non-phagocytic cells, such as human lung-derived epithelial cells, and are internalized through pinocytosis. Filomicelles shrink in length over time, with the rate of shortening decreasing as they shorten. Filomicelles loaded with paclitaxel show increased apoptosis and tumour shrinkage, with longer filomicelles being more effective. Filomicelles have potential as drug delivery vehicles due to their long circulation time and ability to deliver drugs to tumours. The study highlights the effects of shape in biological systems at the nanoscale and the potential of filamentous carrier systems for drug delivery.This study compares the transport and trafficking of flexible filaments (filomicelles) with spherical particles of similar chemistry in rodents. Filomicelles, which are polymer micelle assemblies, persist in the circulation up to one week after intravenous injection, which is about ten times longer than their spherical counterparts and more persistent than any known synthetic nanoparticle. Under fluid flow conditions, spheres and short filomicelles are taken up by cells more readily than longer filaments because the latter are extended by the flow. Preliminary results show that filomicelles can effectively deliver the anticancer drug paclitaxel and shrink human-derived tumours in mice. These findings suggest that long-circulating vehicles need not be nanospheres and provide insight into possible shape effects of natural filamentous viruses. Filomicelles are cylindrical micelles self-assembled from block copolymers with hydrophilic and hydrophobic chains. They have a hydrophilic chain of polyethylene glycol (PEG) and a hydrophobic chain of either inert polyethylene or biodegradable polycaprolactone. Filomicelles persist in the circulation for up to one week, with longer filomicelles persisting longer. Filomicelles enter cells under static conditions but are opposed by flow. The length of filomicelles affects their ability to enter cells and their fragmentation. Degradable filomicelles of OCL3 show a similar but more sustained decrease in length due to hydrolysis. Filomicelles loaded with paclitaxel shrink tumours, with longer cylinders proving more effective at a given dose. Filomicelles interact with phagocytic cells, with longer filomicelles being less likely to be taken up due to flow-induced extension. Filomicelles can enter non-phagocytic cells, such as human lung-derived epithelial cells, and are internalized through pinocytosis. Filomicelles shrink in length over time, with the rate of shortening decreasing as they shorten. Filomicelles loaded with paclitaxel show increased apoptosis and tumour shrinkage, with longer filomicelles being more effective. Filomicelles have potential as drug delivery vehicles due to their long circulation time and ability to deliver drugs to tumours. The study highlights the effects of shape in biological systems at the nanoscale and the potential of filamentous carrier systems for drug delivery.