Treatment of cancer is one of the crucial enigma for scientific world and that’s why much effort needs to be put in place for the resolution of this challenge in alternative ways. Autophagy is believed to have an important role in tumor development and progression. The natural polyether antibiotics might be important chemotherapeutic agents to cure cancer by modulating autophagy. The primary goal of this study was to investigate the cytotoxic effects and autophagic mechanism of actions of three polyether antibiotics, one of which was a new secondary metabolite isolated from the marine Streptomyces cacaoi. The effects of these polyether antibiotics were investigated along with previously known autophagy modulators from the same group (Monensin). To achieve this goal, cytotoxicities of these polyether type compounds on three different type of cancer cell lines along with two healthy cell lines were investigated followed by a search to reveal the effects of these compounds on autophagy in cancer cell lines. Methodology of this study consists of mammalian cell culturing, cytotoxicity screening, staining and quantification of acidic compartments inside the cells and studying different autophagy markers along with other associated proteins under various conditions by using Western blotting. This study revealed that the tested polyether antibiotics were autophagy inhibitors as well as inducers of apoptosis in cervical, colorectal and prostate cancer cells. The obtained results will be of significance for the field of anticancer drugdevelopment; however, before one places these secondary metabolites as p