Zinc oxide (ZnO) particle induced cytotoxicity was dependent on size, charge and solubility, factors which at sublethal concentrations may influence the activation of the human monocytic cell line THP1. ZnO nanoparticles (NP; average diameter 70nm) were more toxic than the bulk form (<44µm mesh) and a positive charge enhanced cytotoxicity of the NP despite their relatively high dissolution. A positive charge of the particles has been shown in other studies to have an influence on cell viability. Centrifugal filtration using a cut off of 5kDa and Zn element analysis by atomic absorption spectroscopy confirmed that exposure of the ZnO particles and NP to 10% foetal bovine serum resulted in a strong association of the Zn2+ ion with the protein. This association with protein may influence interaction of the ZnO particles and NP with THP1 cells. After 24h exposure to the ZnO particles and NP at sublethal concentrations there was little effect on immunological markers of inflammation such as HLA DR and CD14, although they may induce a modest increase in the adhesion molecule CD11b. The cytokine TNFa is normally associated with proinflammatory immune responses but was not induced by the ZnO particles and NP. There was also no effect on LPS stimulated TNFa production. These results suggest that ZnO particles and NP do not have a classical proinflammatory effect on THP1 cells.