The survival of blood neutrophils labeled in vivo was studied in the calf. Disappearance of labeled neutrophils from the blood of calves was followed after a period of cross circulation with their chimeric, immunologically tolerant twins, which had been given tritiated thymidine 6½ days previously. Under these conditions, neutrophils were shown to leave the blood in a random exponential fashion, with half-disappearance times of between 6.4 and 7.5 hr. Hydrocortisone given to one calf 48 hr after cross circulation caused a neutrophilic leukocytosis, during which substantial numbers of labeled neutrophils reappeared in the blood.