In CHO (Chinese Hamster Ovary) cell calcium flux assays, the presence of high organic anion transporter activity can result in poor retention of fluorescent calcium dyes, such as Fluo-3 and Fluo-4 within the cells. To enhance the retention of these dyes and achieve optimal signal sensitivity, it is necessary to include probenecid in the assay protocol. Probenecid is a compound which inhibits organic anion transporters and prevents the expulsion of fluorescent dyes from the cells. In a standard calcium flux assay for CHO cells using Fluo-3, Fluo-4, or similar commercial kits, a concentration of 2.5 mM probenecid is recommended. It is important to note that probenecid is not required when using certain types of assay kits (e.g. BD™ PBX Calcium Assay Kit).