Nanotoxicity assessment methods for nanoparticles (NPs) such as carbon nanotubes (CNTs), nano-Al(2)O(3), and tridecameric aluminum polycation or nanopolynuclear (nano-Al(13)), particularly lactate dehydrogenase (LDH) assays are reviewed. Our researches on electrochemically monitoring the variations of LDH activity indirectly in the presence of multiwalled carbon nanotubes (MWCNTs), nano-Al(13), and nano-Al(2)O(3) separately to derive toxic concentrations of NPs altering LDH activity by 50% (TC(50)) values are discussed. TC(50) values indicated that the toxicity order was Al (III)> MWCNTs > nano-Al(13) > nano-Al(2)O(3). Zeta potentials (ζ) data of these NPs in the literature proved that the surfaces of these NPs are charged negatively. Negatively charged surfaces might be a main cause in the reduction of LDH activity. Therefore, the classic LDH assays are doubtful to underestimate the nanotoxicities when they are applied to those NPs with negatively charged surfaces. These observations highlight and reconcile some contradictory results at present such as medium-dependent toxicity of NPs among the literature and develop novel analytical methods for evaluation of toxicities of NPs.