Background: In vitro chemosensitivity testing of circulating epithelial tumor cells (CETCs) provides real-time information about the sensitivity of the tumor cells present in the patient and correlates with treatment success. Nevertheless, a fraction of CETCs can survive after conventional chemotherapy and grow into distant metastasis. A subpopulation of CETCs with proliferation activity has the ability to form spheroids in suspension culture. Spheroids exhibit stem cell-like properties and may be responsible for chemo therapeutic resistance. Therefore, the aim of our study was the comparison of the efficacy of chemo therapeutics on CETCs and on spheroids originated from the same individuals. Methods: The enumeration of CETCs collected from patients with solid tumors in clinical stage 1-4 were performed using the maintrac method. Subsequently, viable CETCs were cultured in suspension culture system allowing for spheroid formation. To evaluate the cytotoxic effect CETCs and spheroids were exposed to anticancer drugs in short time culture in different concentrations and for different periods of time. Results: Theresponse to chemotherapeutics was different between CETCs and spheroids. In contrast to CETCs, spheroids from the same patients were significantly more chemoresistant. Whereas active drugs led to membrane permeability in single CETCs with subsequent staining of the nuclei with propidium iodide, the same drugs led to disintegration of tumorspheres with destruction of part of the cells but often part of the cells in the spheres were able to survive. Epirubicin and, interestingly, and especially salinomycin, a polyether ionophore antibiotic isolated from Streptomyces albus, showed the best effects. Docetaxel, cyclophosphamide and 5-Fluoruracil showed almost no cytotoxic effects onto the cells in the spheres. Conclusions: Our results show, for the first time that stem cells circulating in peripheral blood, capable of forming spheroids are way more resistant to anticancer drugs than the remnant circulating tumor cells. We, furthermore, demonstrate that salinomycin efficiently destroys spheroids cultured from CETCs, strengthening its role as promising anti-cancer therapeutic.