β-Lactamase tagged clones faithfully report real-time transcriptional responses in living cells. A β-lactamase tagged library was created in human endothelial cells. Fluorescent activated cell sorting (FACS) was used to isolate cell clones that responded to stimulation by a proinflammatory cytokine with an increase in β-lactamase expression. Kinetic profiling plots are shown for two of the cell lines isolated in which response is the ratio of blue fluorescent emission (460 nm) compared with green fluorescent emission (530 nm). Cells were stimulated with either interleukin-1β (IL-1β) or tumor necrosis factor-α (TNFα) at a concentration of 10 ng/ml. Results plotted are the average fluorescent ratio plus or minus s.d. for four parallel experiments. Digital fluorescent images are shown of clone E424 either unstimulated (C) or stimulated with IL-1β for 6 hr (D) and loaded with the CCF2 β-lactamase substrate. The genes tagged in these two clones were identified by RACE as CCA3 (GenBank no. AB000216;A) for E395 and RelB (GenBank acession no. M83221, B) for E424.