Leukocyte kinetics were studied in two individuals with marked eosinophilia. Blood leukocytes, labeled in vitro with Na251CrO4, disappeared from the circulation at different rates in the two subjects during the first eight hours after infusion. In one, label disappeared exponentially with a T/2 of 4.5-5.0 hours; in the other, the level of radioactivity remained constant. In both, however, a significant rise in radioactivity appeared during the second 24 hours, suggesting recirculation of the eosinophils. Blood leukocyte DNA-32P labeling was similar in both subjects and closely resembled neutrophil DNA-32P labeling observed in normal individuals. Hydrocortisone infusion produced an exponential decrease in circulating eosinophils in one subject and, for reasons unknown, a linear decrease in the other; the drug exhibited no effect on eosinophils in vitro.