Blazein was isolated from mushroom (Agaricus blazei Murrill) and identified by Mass and 1H-NMR as blazein. The effect of blazein on the DNA of human various cancer cells was investigated. DNA fragmentations by blazein to oligonucreosomal-sized fragments, a characteristic of apoptosis, were observed in the human lung LU99 and stomach KATO III cancer cells. The DNA fragmentations by blazein were observed from day 2 (KATO III cells) or day 3 (LU99 cells) after the addition of blazein to the culture cells. These findings suggest that growth inhibition by blazein results from the induction of apoptosis by the compound.