At the beginning of his life, mankind started to use plants for therapeutic purposes. Many drugs used in modern medicine are also derived from plants. Although plants are used in the field of alternative medicine (used in place of medical treatment) today, the correct thing is to use them in the field of complementary medicine. The use of complementary medicine is based on the use of the plant together with medical treatment [1]. If plants are not used consciously for therapeutic purposes, human health may deteriorate while trying to be treated. Because there are many important parameters such as the application dose, whether the right plant is used, its side effects, whether it will trigger other diseases, and its toxic effects. For this reason, scientists who work on the potential of plants to be medicine have important duties. Tribulus terrestris L. is a plant known in Turkey as “deve çökerten”, “çoban çökerten” and “çarık dikeni”. The plant is so called because its fruit has horn-shaped spikes. The medicinal and pharmaceutical importance of the plant is based on its steroidal saponin content. There are limited studies on the effect of T. terrestris L. extracts, which have been reported to have low cytotoxicity on healthy cells in many studies, on different cancer cells [2]. In this context, in this study, cytotoxic effects of ultrapure aqueous extracts of Tribulus terrestris L. obtained by ultrasonic wave-assisted extraction method were investigated on the colon (DLD-1), breast (MCF-7), and prostate (PC-3) cancer cells and human peripheral blood lymphocyte cells (PBMC) were investigated by the MTT Method. Extracts were prepared in the concentration range of 7.81-1000 µg/mL. As the concentration of the plant increased, it was determined that human peripheral blood lymphocyte cell viability increased. In this determined concentration range, the plant caused a moderate cytotoxic effect on the colon (DLD-1), breast (MCF-7), and prostate (PC-3) cancer cells.