The use of herbs has been the mainstay of treatment for a variety of human illnesses and is an essential part of culturally-based healing traditions in many societies and countries. Also, herbs, including Chinese herbs, are being incorporated as remedies for disease management and treatment in Western countries. In Traditional Chinese Medicine (TCM), herbal prescriptions are most frequently given to patients as complex formulations containing multiple herbs. Notably and unwittingly, this approach amounts to the administration of several chemical entities at once; the underlying theory is that interactions among the chemicals present in different herbs in a formula exert synergistic pharmacodynamic actions and neutralize the adverse effects and toxicities of specific individual chemicals. The effectiveness and mechanisms of this approach have not been well investigated or understood. A primary interest of this laboratory is to obtain experimental evidence that supports the fundamental mechanistic theme for the combinatorial herbal strategy described above and its potential application in preventing and treating breast cancer (BCa). In this study, we investigated the effects of 70% ethanolic extracts prepared from medicinal mushroom extract denoted I'm-Yunity and Danshen (Salvia miltiorrhizae Binge), alone and in combination, using MCF-7 cells as an in vitro model of estrogen receptor positive (ER+), low invasive BCa. Combination of I'm-Yunity and Danshen (referred to as I'm-Yunity-Plus) suppressed clonogenicity to a comparable degree as Danshen alone; both being significantly more active than I'm-Yunity. However, extract of Danshen was more active in inhibiting MCF-7 cell growth than I'm-Yunity-Plus. In comparison, I'm-Yunity elicited less growth inhibition. Flow cytometric analysis showed that I'm-Yunity-Plus induced partial block of G1/S transition in MCF-7 cells, whereas Danshen slowed down cell progression from G1/S into G2/M phases of the cell cycle. Treatment by I'm-Yunity did not affect cell cycle progression in MCF-7 cells; however, it promoted active induction of apoptosis. In addition, treatment with Danshen alone resulted in a pronounced reduction in the expression of Rb, cyclin D1, and p53, and also led to a diminution of p65 and p50 forms of NF-kappaB. The pronounced suppressive effects of Danshen on expression of the aforementioned genes were largely attenuated in cells treated with I'm-Yunity-Plus suggesting that ingredients in Danshen must have interacted with those in I'm-Yunity as to culminate in neutralization of the gene suppressive effects of Danshen. Additional support for such interactions was obtained by targeted cDNA array analysis using human tumor metastasis and BCa/ER signaling gene arrays. Taken together, our results are consistent with the interpretation that interaction exists between Danshen and I'm-Yunity and that I'm-Yunity-Plus may have efficacy in the treatment of BCa, particularly for patients with ER+ status.