The immune system is a vital physiological component that affords animals protection from disease and is composed of innate and adaptive mechanisms that rely on cellular and dissolved components. The serum complement system is a series of dissolved proteins that protect against a variety of pathogens. The activity of complement in serum can be determined by its ability to lyse red blood cells in vitro. Here, we describe a modification of a standard complement hemolysis assay that makes an interesting and informative laboratory exercise suitable for a variety of courses including physiology. (Contains 2 tables and 1 figure.)