Chemotactic activity for human peripheral blood leucocytes was generated by the action of thrombin on human fibrinogen, a reaction known to release low molecular weight fibrinopeptides. Following gel filtration of the clot supernatant most of the activity eluted at the sameKdas synthetic fibrinopeptides. Fibrinopeptides present in the clot supernatant were then identified and separated by high voltage electrophorcsis in two dimensions. Chemotactic activity was a property of fibrinopeptide B and not of fibrinopeptides A, AP or AY. Supernatants prepared with contortrix venom, which cleaves the B peptide, were also chemotactic whereas no activity was present in supernatants prepared from Arvin venom which cleaves peptides A, AP and AY. Synthetic fibrinopeptide B and a B analogue, 1‐glutamic acid, were found to be chemotactic but not synthetic fibrinopeptide A. Thus chemotaxis of human leucocytes represents a further biological activity of fibrinopeptide