Granulocyte kinetics after standardized, sublethal, third-degree burn injury were studied in mice in vivo and in vitro using tritiated thymidine. Total haematopoietic activity was determined and autoradiographic analyses were made 2 hours to 30 days after the burn injury. During the first few hours postburn there was a 50% inhibition in the total incorporation of ³H-TdR in DNA and a 30% suppression in the labelling index for late proliferating granulocytes. The results suggest a selective destruction of bone marrow tissue, exhaustion of the marrow granulocyte reserve, inhibition of cell production and aimless distribution of cells in the tissues followed by rapid destruction, and subsequent release of cellular contents. Strong leukocytosis during the first days postburn reflects the leukocyte release. The burned organism is short of granulocytic phagocytes from the third day post-burn, and although the bone marrow quickly overcomes the initial suppression, these cells are available to the organism only after the first week postburn. The results obtained in vitro indicated that the inhibition of granulocyte production and the accelerated release are not entirely locally induced but may involve humoral regulatory mechanisms.