Cell tracking by magnetic resonance imaging (MRI) is an emerging technique that typically requires the use of MRI contrast agents (CAs). A MRI CA for cellular imaging should label cells efficiently at potentially safe concentrations, have high relaxivity, and not affect the cellular machinery. In this article, we report the cytotoxicity, cytocompatibility, and cell labeling efficiency in NIH/3T3 fibroblasts of novel, single-walled carbon nanotubes synthesized using gadolinium nanoparticles as catalysts (Gd-SWCNTs). Cells incubated with the Gd-SWCNT showed a dose- (50–100 µg/mL nanotube concentration) and time- (12–48 h) dependent decrease in viability. 30% cell death was observed for cells incubated with Gd-SWCNTs at the maximum dose of 100 µg/mL for 48 h. Cells incubated with the Gd-SWCNTs at concentrations between 1–10 μg/mL for 48 h showed no change in viability or proliferation compared to untreated controls. Additionally, at these potentially safe concentrations, up to 48 h, the cells showed no phosphatidyl serine externalization (pre-apoptotic condition), caspase-3 activity (point of no return for apoptosis), genetic damage, or changes in their division cycle. Localization of Gd-SWCNTs within the cells was confirmed by transmission electron microscopy (TEM) and Raman microscopy, and these results show 100% cell labeling efficiency. Elemental analysis also indicates significant uptake of Gd-SWCNTs by the cells (108–109 Gd3+ ions per cell). Finally, T1-weighted MRI at 3 T of Gd-SWCNT-labelled cells show up to a four-fold increase in MR signal intensities as compared to untreated cells. These results indicate that Gd-SWCNTs label cells efficiently at potentially safe concentrations, and enhance MRI contrast without any structural damage to the cells. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 101A: 3580–3591, 2013.