Transfection efficiency at high serum concentrations of DOTAP (A) or DC (B) DNA complexes formed at different ratios of lipid to protamine sulfate to DNA. One μg luciferase expression plasmid was mixed with the indicated amount of protamine sulfate prior to addition of the lipid. Complexes were incubated with two volumes of serum for one hour and added to 293A cells. One day later the luciferase activity of the cell extracts was determined. RLU: relative light units. The mean and the standard deviation (error bars) of triplicates is shown.