We quantify here, for the first time, the intracellular uptake (J774A.1 murine macrophage cells) of gadolinium-loaded ultra-short single-walled carbon nanotubes (gadonanotubes or GNTs) in a 3 T MRI scanner using R2 and R2* mapping in vitro. GNT-labeled cells exhibited high and linear changes in net transverse relaxations (ΔR2 and ΔR) with increasing cell concentration. The measured ΔR2* were about three to four times greater than the respective ΔR2 for each cell concentration. The intracellular uptake of GNTs was validated with inductively coupled plasma optical emission spectrometry (ICP-OES), indicating an average cellular uptake of 0.44 ± 0.09 pg Gd per cell or 1.69 × 109 Gd3+ ions per cell. Cell proliferation MTS assays demonstrated that the cells were effectively labeled, without cytotoxicity, for GNTs concentrations ≤28 µM Gd. In vivo relaxometry of a subcutaneously-injected GNT-labeled cell pellet in a mouse was also demonstrated at 3 T. Finally, the pronounced R2* effect of GNT-labeled cells enabled successful in vitro visualization of labeled cells at 9.4 T. Copyright © 2010 John Wiley & Sons, Ltd.