The stability and constant environment of RX1's signal (crystallising fluorophore, protected from interferences), its low background (ESIPT quenched probe) and low crosstalk (high Stokes shift), and its ability to directly generate a readout, were promising features. We therefore set out to develop the first cellular quantitative HTS (qHTS) assay for TrxR1 inhibitors, using RX1, and performing pilot screening with the 1280-compound LOPAC1280 library. LOPAC is intended to cover drugs and drug-like scaffolds with much comparative data on target selectivity, potency, cellular and in vivo bioactivity, without focus on any particular mechanism of action. The LOPAC library was used in the only previously reported qHTS enzymatic TrxR assay (in 2011), and there is 82% overlap in composition between the 2011 and 2021 versions. Cellular screening using RX1, optimized for HTS with 1536-well plates (assay volume 6 µL , 500 cells/well) over 4 h run-time (Table ), reproduced the strong performance seen in 96-well format (Fig. ). Untreated vs no-cell controls gave a 7-fold raw signal to background ratio [S:B] without background compensation, with a high Z' value of 0.64, suitable for HTS (Fig. ). RX1 signal was linear over the assay time, and linearly reflected turnover rate (varied RX1 concentration or cell count; Fig. ). Pre-incubating cells for 1 h with reference inhibitors prior to RX1 gave concentration-dependent inhibition consistent with reported values (Fig. ).