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Despite the pressing need to noninvasively monitor transplanted cells in vivo with fluorescence imaging, desirable fluorescent agents with rapid labeling capability, durable brightness, and ideal biocompatibility remain lacking. Here, phosphorylcholine-coated near-infrared (NIR) fluorescent semiconducting polymer nanoparticles (SPNs) are reported as a new class of rapid, efficient, and cytocompatible labeling nanoagents for in vivo cell tracking. The phosphorylcholine coating results in efficient and rapid endocytosis and allows the SPN to enter cells within 0.5 h in complete culture medium apparently independent of the cell type, while its NIR fluorescence leads to a tissue penetration depth of 0.5 cm. In comparison to quantum dots and Cy5.5, the SPN is tolerant to physiologically ubiquitous reactive oxygen species (ROS), resulting in durable fluorescence both in vitro and in vivo. These desirable physical and physiological properties of the SPN permit cell tracking of human renal cell carcinoma (RCC) cells in living mice at a lower limit of detection of 10 000 cells with no obvious alteration of cell phenotype after 12 d. SPNs thus can provide unique opportunities for optimizing cellular therapy and deciphering pathological processes as a cell tracking label.