Breast cancer is the most commonly diagnosed cancer among women. Currently, there is no effective therapy for malignant estrogen-independent breast cancer. In our study, we used hydrogen peroxide, a well-known strong oxidative reagent capable of activating the nuclear factor kappa B (NF-kappaB) transcription factor. The IC50 value of the culinary-medicinal Shaggy Inc Cap mushroom Coprinus comatus culture liquid crude extract on MCF7 cell viability was found to be as low as 76 microg/mL, and the IC50 value of C. comatus ethyl acetate extract was only 32 microg/ mL. Our results also showed that both extracts significantly affected IkappaBalpha phosphorylation in a dose-dependent manner. The effect of ethyl acetate extract was comparable to the effect of curcumin, a known NF-kappaB pathway inhibitor, and seemed to be the most active inhibitor of H2O2-dependent IkappaBalpha phosphorylation. In addition, the data obtained showed that only ethyl acetate extract inhibited the activity of IKK complex, at close to 90% as compared to the control of the untreated sample. These results suggest that C. comatus contains potent compounds capable of inhibiting NF-kappaB function and also possibly acts as an antitumor agent.