You might try removing the media that contains the transfection complex 4-6 hours post transfection and replace it with growing culture media. If this doesn’t help with cell viability, you might need to do a small experiment to optimize a few key parameters that can influence the performance of your transfection. Cell density can be an important factor for cell viability and transfection. If cell density is too low at the time of transfection, then a big drop in viability may be seen. Also, ensure that high quality DNA is used. We suggest to test two different cell densities, two to four recommended lipid doses and two DNA doses.