Nonradioactive cesium, as an analogue of potassium, has been used to label autologous red blood cells for determination of the red cell volume in man. The initial and the equilibration concentrations of cesium are assayed by fluorescent excitation analysis (FEA), using a 600 mCi 241Americium source and a Si(Li) detector with a 1024-channel analyzer. Comparative studies with 51Chromium in 13 rabbits showed good correlation, but the intracellular cesium concentration achieved by simple incubation with 2.6 per cent cesium chloride solution was too low to be of practical value in humans. Incubation of the human red blood cells with 50 mug per milliliter of Nystatin in 2.6 per cent cesium chloride opened reversible "pores" in the red cell membrane which permitted high intracellular cesium labeling without demonstrable red cell damage. The cesium red cell volumes in 11 random human subjects differed from the 51Chromium red cell volumes by only 0.2 +/- 4.5 per cent and 2.5 +/- 7.6 per cent at blood sampling times of 10 minutes and 40 minutes, respectively. Blood cesium levels fell with a clearance half-time of 31.5 hours in 4 rabbits, and 2.4 days in 1 normal human. Fluorescent excitation analysis of cesium-labeled autologous red blood cells permits accurate determination of the red cell volume in man without associated patient radiation, thus making the procedure much more acceptable for children, pregnant women, normal volunteers, and for repeated studies in the same individual.