In the Huaier-treated groups, a downregulation in the expression of Bcl-2 and pro-caspase-3, an upregulation in the expression of Bax and cleaved caspase-9 and-3 and a decrease in the mitochondrial membrane potential indicated that Huaier induced the apoptosis of fibrosarcoma cells through the mitochondrial pathway. Here, CASP9 is linked to fibrosarcoma.