Leukotriene B4 (LTB4 isomer III), which promotes the movement and aggregation of leucocytes in vitro also stimulates the chemo-attraction of leucocytes and their adherence to vascular endothelium in vivo. These effects were observed directly in the hamster cheek pouch preparation and on histological examination of sections from the rabbit mesentery. Intravenous injection of LTB4 (isomer III) into the rabbit resulted in a profound but transient neutropenia. Intradermal injection of LTB4 (isomer III) in the rabbit produced a rapid accumulation of neutrophils and this effect was also observed in skin chambers applied over abrasions on the rabbit back or the human forearm.