This laboratory exercise demonstrates the use of an immunoassay for studying kinase enzyme activity in living cells. The advantage over the classical method, in which students have to isolate the enzyme from cell material and measure its activity in vitro, is that enzyme activity is modulated and measured in living cells, providing a more realistic picture of cell metabolism. With this experiment, students learn not only how to study protein kinases but also how to study the MAP kinase signaling pathway, how to set up their own experiments, and how to study the effects of pharmacologically important ligands on signaling in a particular metabolic pathway. Signaling with protein kinases, as described here, is a good example of how to study molecular mechanisms of signaling in general.