The magnetic field mediated capture of 10nm diameter superparamagnetic nanoparticles, in the form of agglomerates of mean diameter 330 and 580nm, from microcapillary flows has been observed and modelled. The steady state thickness of the captured layer in microcapillaries of diameter 400-800pm could be predicted for both the 330 and the 580nm diameter agglomerates at flow rates of between 0.1 and 0.4mlmin-1. The model provides insight into blockage formation at a constant flow rate as a precursor to the prediction of thrombotic embolism in magnetic directed therapies. Capillary constriction was particularly acute for the 580 nm agglomerates in large microcapillaries (800 pm) with flow rates of 0. 1 ml min-1. From this model, agglomerates of diameter 33.0 nm or less offer the potential for minimal microcapillary occlusion in a range of flow rates. (c) 2008 Elsevier Ltd. All rights reserved.