Aluminum nanoparticles (Al NP) have been used in applications as diverse as drug delivery, material surface coatings and an ingredient for solid rocket fuel in military explosives and artillery. Although Al NP are used in many civilian and military applications, the health and safety implications of these nanosize particles are not known. To understand the interactions and biological activity of Al NP in human cells, cultured human neonatal epidermal keratinocytes (HEK) were exposed for 24 h to 50 and 80 nm Al NP in concentrations from 4.0 to 0.0004 mg ml−1 to assess the cytotoxicity and inflammatory potential. UV–Vis measurements and nanoparticle controls revealed that the Al NP interact with the assay dyes. Viability did not decrease in HEK exposed to both the 50 and the 80 nm Al NP at all treatment concentrations with MTT, CellTiter 96® AQueous One (96 AQ) and alamar Blue® (aB) viability assays. The 96 AQ and aB assays interact with the Al NP less than MTT, and proved to be the best assays to use with these Al NP. TEM depicted Al NP localized within the cytoplasmic vacuoles of the cells. Cytokine data was variable, indicating possible nanoparticle interactions with the cytokine assays. These studies illustrate the difficulties involved in assessing the biological safety of nanomaterials such as Al NP due to media- and temperature-dependent particle agglomeration and nanoparticle interactions with biomarkers of cytotoxicity. Copyright © 2009 John Wiley & Sons, Ltd.