The Tetrahymena trans-splicing ribozyme can edit RNA in a sequence-specific manner, but its efficiency needs to be improved for any functional rescues. This communication describes a simple method that uses a bacterial enzyme beta-lactamase to report trans-splicing activity of Tetrahymena ribozyme in single living mammalian cells by fluorescence microscopy and flow cytometry. This enzyme-based single-cell detection method is highly sensitive and compatible with living cell flow cytometry, and should allow a cell-based systematic screening of a vast library of ribozymes for better trans-spliced ribozyme variants.