We have developed a general experimental strategy that enables the quantitative detection of dynamic protein–protein interactions in intact living cells, based on protein-fragment complementation assays (PCAs). In this method, protein interactions are coupled to refolding of enzymes from cognate fragments where reconstitution of enzyme activity acts as the detector of a protein interaction. We have described a number of assays with different reporter readouts, but of particular value to studies of protein interaction dynamics are assays based on enzyme reporters that catalyze the creation of products, thus taking advantage of the amplification of signal afforded. Here we describe protocols for one such PCA based on the enzyme TEM β-lactamase as a reporter in mammalian cells. The β-lactamase PCA consists of fusing complementary fragments of β-lactamase to two proteins of interest. If the proteins interact, the fragments are brought together and fold into active β-lactamase. Here we describe a protocol for this PCA that can be completed in a few hours, using two different substrates that are converted to fluorescent or colored products by β-lactamase.