Leukotriene B4 isomer III is released from polymorphonuclear leucocytes, monocytes, eosinophils and macrophages in vitro and has been detected and measured in human synovial fluid in vivo. Its most prominent biological activities are to induce the aggregation of and to stimulate the movement (chemokinesis and chemotaxis) of leucocytes in vitro and it acts as a cytotaxin in vivo. In addition, it increases vascular permeability in vivo when administered together with the vasodilator, PGE2.