Aqueous extract of pineapple (Bromelain) is used in medicine. Bromelain has anticancer effect on leukemia, lymphoma, sarcoma and melanoma. The purpose of this study was to evaluate the anticancer effect of bromelain on mouse breast cancer (4T1) cells under in-vitro conditions. Methods: Mouse breast cancer (4T1) cells were incubated with different concentrations of the bromelain for 2, 24, 48 and 72 hours. The growth-inhibitory was investigated via 3 (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide (MTT) assay. Kruskal-Wallis and Dunn’s tests were used to evaluate the statistical significance of the difference between the experimental and control groups. Findings: Bromelain significantly inhibited proliferation of 4T1 cells in a concentration-dependent manner. In all concentrations, cell viability decreased with respect to the control incubated cells in the absence of extract (P < 0.05). Bromelain exerted highest antiproliferative effects on 4T1 cells for concentrations of 100, 200, 300, 400 and 600 µg/ml (P < 0.05) for 24, 48 and 72 hours, while cell viability increased for times shorter than 2 hours. The half maximal inhibitory concentration (IC50) value of 142.6 g/ml was obtained from treating 4T1 cells. Conclusion: Results showed that bromelain has cytotoxic effects on the growth and proliferation of 4T1 cells for 24, 48 and 72 hours of incubation. This study is ongoing to assess the extract mechanism of action
