Breast cancer is a disease in which cells in the breast mutate and turn malignant, where the mutated cells spread to other parts of the body and develop into secondary cancer. In 2018, there were 58, 256 new cases of breast cancer. With improper handling, breast cancer can be lethal, where there are 22, 692 deaths in 2018 alone. One type of cancer with the most sufferers is estrogen, progesterone, and glucocorticoid receptors. Research on cancer treatment continues by looking for potential substances as new drugs. Toxins from animals that are cytolytic can trigger apoptosis in cancer cells. Some studies suggest that the lethal factor of the stonefish poison, Stonustoxin (SNTX), has cytolytic properties that damage cell membranes. This study discusses the stonefish (Synanceia horrida) venom as a breast anticancer agent. Stonefish poison was milked using a syringe, and the SNTX purification was carried out by the FPLC method using a 5 ml HiTrap Q H.P. substantial anion column. The percentage of inhibition of MCF-7 breast cancer cells will be measured using the MTT Assay method. As a result, MCF-7 breast cancer cells could be inhibited with IC50 values of 8.2771 μg/ml.