Dye exclusion testing is a simple and rapid method to determine the ratio of viable and nonviable cells in a cell suspension, a key indicator of cell culture health and competency for transformation. Viable cells with intact membranes exclude dyes such as trypan blue, propridium iodide, and eosin; whereas, nonviable cells absorb them. The ratio of viable and nonviable cells can be ascertained manually using a hemacytometer and microscope, or the ratio can be automatically determined using image-based cell viability analyzers such as the Vi-CELL? or Vi-CELL Blu?.