Inhibition of cell shedding from the primary tumour, whereby metastases at distant sites within the body are formed, could be exploited to avoid metastasis and delay the progression of the cancer disease. Herein, we investigated the effects of polyphenols and NADPH oxidase (NOX) inhibitors on cell shedding from multicellular tumour spheroids (MTS) of the murine mammary cancer cell line 4T1, cell invasion into embryonic stem cell- derived embryoid bodies (EB), generation of reactive oxygen species (ROS) and expression of matrix metalloproteinase 9 (MMP9). With increasing tumour spheroid growth MMP9 expression was upregulated and cells detached from MTS and formed subspheroids. Upon incubation with polyphenols, tumour growth was arrested and cell shedding was totally abolished. Polyphenol treatment decreased ROS generation and downregulated MMP9 expression. The ROS level was likewise decreased following NOX inhibitor treatment. Furthermore, polyphenols and NOX inhibitors suppressed invasion of tumour cells into vascularized EBs. Our data suggest, that polyphenols inhibit cell shedding and invasion by their anti-oxidative capacity and downregulation of MMP9 expression.