Fig. 2. Fluorescence intensity (FI) ratios for cells versus bath (circles) and lumen versus bath (triangles) as functions of the concentration of Texas Red in the saline bathing isolated cricket Malpighian tubules. An FI ratio for each site was calculated from the mean of five FI measurements in the bathing saline, cells and lumen. Each data point is the mean (± S.E.M.) ratio for 5–6 widely separated regions in three tubules.