In this experiment, students prepare a protein extract from cow heart. They then determine the molecular weights of major contractile proteins by comparing their migration on SDS-polyacrylamide gels to the migration of standard proteins of known size. Students also identify and determine the molecular weight of the major proteins found in milk. This exercise requires 1 three-hour or 2 two-hour lab sessions and a table top centrifuge or microcentrifuge is required. Typical results of the exercise are shown on the gel.