We present a laboratory practice aimed to clarify and discuss the concept of bioindicator molecules in environmental health. It has been implemented as a hands-on laboratory exercise in postgraduate courses in public health (in Brazil) and in a course entitled Biology Applied to Natural Resources Sustainability (in Spain) but can be useful for undergraduate studies. By simple, repeatable, and low cost spectrophotometry methodology, students detect minimal concentrations (nmol) of metallothionein in biological samples obtained from sites with various status of pollution. They can also expose experimental fish or mollusks to heavy metals in aquarium conditions and then detect metallothionein. This method is excellent for didactic purposes because it can be developed even in modest laboratories with basic equipment for molecular biology.