Ferumoxtran-10, a dextran-coated ultrasmall superparamagnetic iron oxide particle, has the potential to reveal macrophages in vivo using magnetic resonance imaging potentially acting as a marker of inflammatory status. Pending clinical trials, we examined the interactions of Ferumoxtran-10 with human monocyte-macrophages (HMMs) in vitro to assess its safety and lack of pro-inflammatory activity. After 72 h, Ferumoxtran-10 was not toxic at 1 mg/ml and may be only mildly toxic at 10 mg/ml. Viability in cells with a high intracellular Ferumoxtran-10 load was not affected over 14 days. Ferumoxtran-10 did not interfere with baseline or stimulated cytokine (interleukin-12, interleukin-6, tumour necrosis factor-alpha or interleukin-1beta) or superoxide anion production or with Fc-receptor-mediated phagocytosis. Similarly, Ferumoxtran-10 did not induce cytokine production and was not chemotactic. High-resolution electron microscopy and selected-area electron diffraction confirmed the core of Ferumoxtran-10 is composed of crystalline magnetite. Bright field transmission electron microscopy of thin sections demonstrated that Ferumoxtran-10 was retained in lysosomes of HMM for several days. Ferumoxtran-10 is not toxic to HMMs in vitro, does not activate them to produce pro-inflammatory cytokines or superoxide anions, is not chemotactic and does not interfere with Fc-receptor-mediated phagocytosis. Furthermore, extremely high intracellular Ferumoxtran-10 concentrations had only slight or no effects on these key activities.