Transfection of siRNA is a critical step in gene silencing experiments. RNAiFect™ Transfection Reagent, based on a lipid formulation, provides efficient transfection of siRNA into a wide range of eukaryotic cells. Low endotoxin levels and testing for absence of RNase activity ensure reproducible, quality performance in demanding RNAi experiments. It can be used for transfection of a wide range of cell types, including AGS, HEK-293, HeLaS3, Huh-7, HUVEC, NIH-3T3, and PA317 cells. The reagent is provided in ready-to-use form; simply add the reagent to the diluted siRNA, mix, incubate, and pipet the complexes on to the cells. Transfection can be performed in the presence of serum, eliminating the need to remove the complexes from the cells. Formation of transfection complexes occurs efficiently in complete growth medium. In some cell lines, replacing growth medium with Buffer EC-R during complex formation enhances the silencing effect. RNAiFect is also available in bulk for high-throughput experiments.