We demonstrate the feasibility of detecting individual tumor-infiltrating cells in vivo, by means of cellular magnetic labeling and a 1.5 Tesla clinical MRI device equipped with a high-resolution surface coil. Using a recently developed high-temperature superconducting (HTS) surface coil, single cells were detected in vitro in voxels of (60 μm)3 at magnetic loads as low as 0.2 pg of iron per cell. The same imaging protocol was used in vivo to monitor infiltration of ovalbumin-expressing tumors by transferred OVA antigen-specific cytotoxic lymphocytes with low iron load. Magn Reson Med 60:1292–1297, 2008. © 2008 Wiley-Liss, Inc.