1. The erythrocyte Cr51 elution rate was determined in 38 patients with hematologic diseases.
2. In four patients with finite red cell life spans, two exponential Cr51 elution rate constants could be calculated. In the remaining 34 patients, the data were consistent with a single exponential elution rate constant from day 1 to day 30-40 following Cr51 administration.
3. The single elution rate varied from 0.62 to 2.27 per cent per day.
4. In two patients, the chromium elution rates determined on two separate occasions were not significantly different. In a third individual, the chromium elution rate constant was 0.75 per cent per day when the red cell life span was 66 days and 1.07 per cent per day when red cell life span was 79 days.