Nano-scale magnetic materials are increasingly being used in the biomedical field for medical imaging, drug-delivery, magnetic sorting, and therapeutic hyperthermia. In particular, magnetic cell labelling offer attractive possibilities for MRI cell imaging, allowing the tracking of a cell population in living animals. Understanding of the interaction of nanoparticles with cells is then of paramount importance. Here, we characterize the uptake of anionic magnetic nanoparticles by THP1 monocytes and derived macrophages. The incorporation of nanoparticles was assessed by electron microscopy. Cell capture was then quantitated as a function of incubation time and extracellular iron. A simple binding–internalisation mechanism allowed modelling all uptake curves, leading to the affinity constants, the maximal mass that can adsorb on the cell membrane, the internalisation time constants and capacities. Though the adsorption step was comparable for monocytes and macrophages, the latter exhibited a more than ten times higher endocytotic activity. For both cell types, we report an excellent efficiency of the magnetic label, with a maximum load of 6 pgFecell−1 in monocytes and almost 50 pgFecell−1 in derived macrophages. Finally, we demonstrated that endocytosed nanoparticles did not affect differentiation of labelled monocytes into macrophages.