Multiwalled carbon nanotubes (MWCNTs) are highly ordered hexagonal lattices of carbon atoms arranged into cylinders by hydrogen bonding, dipolar forces, hydrophilic or hydrophobic interactions, gravity, and other forces. MWCNTs are synthesized by applying energy to a carbon source, which produces individual or groups of carbon atoms that reassemble into tubes. One of the primary uses of MWCNTs is in nanotube-reinforced polymer composite materials that take advantage of their low-density and high load-bearing capacity. Nanoscale materials were nominated by the Rice University Center for Biological and Environmental Nanotechnology to the National Toxicology Program for toxicologic testing. Because long-term inhalation toxicity and carcinogenicity studies were being conducted on a relatively short, rigid MWCNT, a representative long and thin MWCNT was selected for these studies. Following an evaluation of 24 different long, thin MWCNTs, the 1020 Long Multiwalled Carbon Nanotube (L-MWNT-1020) (Sun Innovations, Fremont, CA) was selected for study based on availability, high purity (97%), and the low amount of residual nickel catalyst (0.52% by weight). The average L-MWNT-1020 nanotube length was 2,600 nm and the average width was 15.3 nm. Because nickel was shown to be tightly bound to L-MWNT-1020, tissue nickel content was measured to determine lung burden. (Abstract Abridged).