Methods and compositions are provided to measure the binding of a test compound to a target peptide by measuring the effect of the compound on the abundance of the target peptide inside a cell. The target peptide may bind the test compound at an active site or an allosteric site, and it has been found that such binding may stabilize the target peptide against cellular degradation. The target peptide will preferably comprise a destabilizing mutation which shortens the half life of the target peptide within the cell, typically a mammalian cell. Test compounds, including small molecules, have been found to stabilize target peptides. Also provided are systems and kits for use in practicing the methods.