Cancer is a genomic functional disease with features of oncogene activation and tumor suppressor inactivation. These genomic features have resulted in the limited effectiveness of conventional therapies and therefore forced considerable efforts to explore new types of anticancer agents. It has been clear that chemically synthesized or in vivo-expressed short interfering RNA (siRNA) can specifically and effectively direct homology-dependent post-transcriptional gene silencing. In the present study, we intended to investigate whether siRNA could suppress the proliferation of human cancer cells through interfering oncogene activities and recovering the functions of tumor-suppressor gene. Single siRNA or combinatorial siRNAs were successfully transfected into HeLa cells, lung adenocarcinoma cells, hepatoma cells, ovarian carcinoma cells, and melanoma cells with cationic lipid complexes. These siRNA molecules not only specifically knocked down their cognate targets such as bcl-2, cdk-2, mdm-2, pkc-alpha, tgf-beta1, H-ras, vegf, and GFP mRNAs, but also effectively suppressed the proliferation of cancer cells to different extents. These data suggest that (1) all these human cancer cells preserve RNAi machinery; (2) chemically synthesized and vector-driven siRNAs can be incorporated into intrinsic RNAi system for silencing target mRNA molecules; and (3) the combination of different siRNAs inhibits the growth and proliferation of cancer cells.