In this exercise students use a simplified version of an immunoassay that relies on the nonspecific binding of proteins to nitrocellulose followed by the specific binding of antibodies to antigen. This antigen–antibody interaction is detected when a chromogenic substrate, catalyzed by the enzyme conjugated to the antibody, produces a color change. The immunoblot is less expensive and faster to perform than plastic-plate-based assays. I have used this assay for over 10 years in undergraduate courses: in ecology to estimate disease incidence; in botany to explore immunological techniques; and in biotechnology as an exercise in product development.