A three-period experiment was developed to help students visualize asymmetric distribution of proteins within membranes. It includes: (1) isolating erythrocyte membranes; (2) differential labeling of intact erythrocytes and isolated erythrocyte membranes with an impermeable fluorescent dye; and (3) separating proteins by polyacrylamide gel electrophoresis and looking at the fluorescent bands. (JN)