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Simple Summary In cases of cancer metastasis, some tumor cells circulate in blood (CTCs) and function as a seed for metastasis. These cells are important for detailed study of the molecular basis of metastasis. However, their few numbers and difficult isolation hinder its analysis. Apheresis, a process used to isolate a specific component of blood, can efficiently separate peripheral blood monocyte (PBMC), whose density is closely like that of CTCs. In the present study we used the dogs as a precious model of human’s cancers to check the safety and feasibility of apheresis and to separate PBMC and infused MCF7 cells using Spectra Optia apheresis machine (Terumo). The process was performed safely to capture PBMC and MCF7. The captured MCF7 cells were regrown up in vitro and characterized. In conclusion, Spectra Optia apheresis machine (Terumo) can be used safely to isolate CTCs from dogs blood with precautions to keep hemodynamic stability. Abstract In patients with solid tumors, circulating tumor cells (CTCs) spread in their blood and function as a seed for metastases. However, the study of CTCs has been limited by their rarity, low frequency, and heterogeneity. The efficient collection of CTCs will contribute to further research of metastatic cancers. Apheresis is a process in which the whole blood of an individual is passed through a machine that isolates a particular constituent and returns the remainder to the circulation. In the present study, we investigated the safety and feasibility of apheresis to separate peripheral blood monocytes (PBMCs), whose density is closely similar to that of CTCs, and to capture intravenously administered human breast cancer cells, MCF7s, from the dogs. No life-threatening events were observed in dogs during the apheresis process. The changes in the hemogram were transient and recovered gradually within a few days after apheresis. During apheresis, 50 mL of PBMCs could be collected from each dog. Notably, a thrombus was formed along the circuit wall during apheresis, which decreased the blood collection pressure. MCF7 cells were successfully captured by the apheresis machine. The captured cells were regrown in vitro and characterized compared with the original cells. In conclusion, apheresis could be safely performed in dogs to isolate CTCs with precautions to maintain hemodynamic stability.