Agaricus blazeiMurill is an edible mushroom distributed in Brazil and presently cultivated in other areas, including Korea, Japan, and China. Its chemical components, including steroids and lectin and various polysaccharides have been widely studied. For this, we used U937/vector and U937/Bcl-2 cells, which were generated by transfection of the cDNA of the Bcl-2 gene. As compared with U937/vector, U937/Bcl-2 cells exhibited a 4-fold greater expression of Bcl-2. Treatment with 0.5 or 4 mg/ml A. blazei Murill for 24 h produced morphological features of apoptosis in U937/vector cells, respectively. This was associated with caspase-3 activation and PARP degradation. In contrast, A. blazei Murill-induced caspase-3 activation and PARP degradation and apoptosis were significantly inhibited by z-DEVD-fmk in U937 cells. Bcl-2 overexpressing cells exhibited sustained caspase-3 activation and expression levels of the Bcl-2 proteins during A. blazei Murill-induced apoptosis. In addition, these findings indicate that Bcl-2 inhibits A. blazei Murill-induced apoptosis by a mechanism that interferes with Bcl-2 degradation and activity of caspase-3 that is involved in the execution of apoptosi.