This program explains the principles of immunoblotting with the aid of computer diagrams and demonstrates how to implement the technique in the laboratory. It explains why attempting to detect the presence of protein samples directly from an electrophoresis gel is impractical, due to diffusion of the protein bands over time and the fragility of the gel itself. The technique of immunoblotting is then illustrated in the laboratory, by showing a method of making a single specific protein visible by using an antibody to that protein. The effect of adding a second antibody with an enzyme attached to produce a colored product when a substrate is added is also shown. (13 minutes)