Dear Editor,With regards to the letter from Dr. Peter H. M. Hoet about our recent publication in TAAP (2006, 217, 252–259), our responses to the letter are as follows: Dr. Hoet suggests that ng/cell should be used as the unit of dosage. The expression of dosimetry unit is a complicated issue. Preliminary histochemical analysis indicates that cell uptake of particles is a small fraction of the applied quantity. Since the cells were exposed to a uniform suspension of nanoparticles, the use of μg/ml units to express the dosage does not seem inappropriate. Many nanoparticle-related cytotoxicity studies have used μg/ml in their in vitro studies (e.g., Sayes et al., Toxicological Sciences 2006 (92) 174–185; Hussain et al., Toxicology in Vitro 2005 (19) 975–983; Gurr et al., Toxicology 2005 (213) 66–73). Of course, the specific contributions of particle mass, particle number, and particle specific surface area to the biological effects of nanoparticles are under vigorous debate and require both theoretical modeling and experiential validation. In any way, this is a topic beyond the scope of our article. The preparation of nanoparticle suspension in our study was described in the last paragraph of the section “Nanoparticles” and in the first paragraph of the section “Cell culture and treatment with SiO2”. We did not assume or state that 45 nm SiO2 and Min-U-Sil have the same oxidative mechanisms of toxicity. Mechanisms of cytotoxic action of nanoparticles can be size dependent; we discussed a size-dependent toxicity study performed by others in the first paragraph of the “Discussion” section. As for the title of the article, A549 is a human bronchoalveolar carcinoma-derived cell line.