Protocol should be viewed as a starting point for systematic optimization of transfection mediated by lipofecting agents. Once a positive signal has been obtained from a transfected plasmid carrying a standard reporter gene, optimal conditions for transfection can be established by systematic variation of parameters such as the initial cell density, the amount and purity of DNA, the media and serum, and the time of exposure of the cells to the cationic-lipid-DNA complex.