Metropolis Monte Carlo modelling combined with image and statistical analyses have been used to simulate clusters of superparamagnetic carrier particles (SCPs) magnetised in a uniform magnetic field during the initial stage of the aggregation process. The dynamics of aggregation is characterised in terms of SCP mean size, size distribution, fractal dimension, distribution of orientations and radial distribution function. The influence of size and concentration of SCPs on the properties of the clusters are investigated. For large (micrometer size) SCPs, chain-like clusters form, oriented around the direction of the magnetic field and having approx. 2.75 and 1.1 mean and fractal dimensions, respectively. Smaller sized SCPs (< 0.1 mum) formed clusters with branched and looped shapes with approx. 3.3 mean and 1.27 fractal dimensions. No preferential orientations against the magnetic field direction have been observed in this latter case. These results are important when considering the design of magnetic separation/filtration devices to optimise the time and efficiency of capture of the SCPs. (C) 2003 Elsevier Science B.V. All rights reserved.