(1) The chemotactic activities of thromboxane B2 (TxB2), PGE2, PGF2alpha, the 15-oxo, 15-oxo-13,14-dihydro and 13,14-dihydro metabolites of PGE2, PGF2alpha, and a metabolite of TxB2 for polymorphonuclear leucocytes (PMN) have been investigated. (2) Thromboxane B2 increased the directional migration of rat peritoneal PMN at a concentration of 2.0 micrograms/ml and of human peripheral neutrophils at a concentration of 0.5 microgram/ml. (3) Neither PGE2, PGF2alpha nor their metabolites showed chemotactic activity for rat peritoneal PMN. (4) PGF2alpha and 15-oxo-13,14-dihydro-thromboxane B2 showed no chemotactic activity for human peripheral PMN. (5) The possible role of thromboxane B2 in inflammation is discussed.