A necessary step in all small interfering RNA (siRNA) library screens is introduction of the siRNA into cells. We describe the use of a commercially available glyceraldehyde 3-phosphate dehydrogenase enzymatic assay that is capable of simultaneously assessing the efficiency of siRNA delivery into cells and the lipid toxicity. This assay has been modified to work in 384-well plates using reverse transfection. The assay is fast, inexpensive, and quantitative. Conditions identified as optimal using this technique have been employed successfully in library screens.