Mesenchymal stromal cells (MSCs) present a potential therapy for disorders of immune dysregulation, but must be better understood in terms of optimal cell preparation, labeling and tracking within the body, as well as general safety following single and repeated administration. This thesis investigated the use of ultra-small superparamagnetic iron oxide (USPIO) nanoparticles tagged with rhodamine B for labeling mesenchymal stromal cells for study in the horse. Ten allogeneic equine cord blood-derived mesenchymal stromal cell lines were used to investigate MSC doubling times, labeling efficiency, and post-thaw cryopreservation viability of labeled cells following USPIO labeling. Results indicated that MSCs can be effectively labeled with USPIO, and that stained cells have excellent post-thaw viability but show longer doubling times under standard culture conditions. To explore clinical safety and to track MSCs within the body following intravenous administration, mesenchymal stromal cells from five allogeneic cord-blood donor sources were labeled with USPIO and approximately 100 million MSCs per dose were injected intravenously through the right jugular vein of six healthy, adult mares on two occasions 35 days apart. Serial hematology and serum biochemistry profile data, as well as lymphocyte immunophenotyping and crossmatching for antibody production, were performed. Bronchoalveolar lavages and lung biopsies were done to track mesenchymal stromal cells within the alveoli and lung parenchyma following intravenous injection. This dosing regime did not elicit any severe hypersensitivity reactions or induce clinically relevant changes in the hematology parameters studied. Mild alterations in AST, GLDH and serum amyloid A (SAA) were seen. The CD4+:CD8+ lymphocyte ratio showed a decreasing trend over time, and minimal anti-MSC serum antibodies were detected.