Chemotactic activity for human peripheral blood monocytes was generated by the action of thrombin on human fibrinogen, a reaction known to release low molecular fibrinopeptides. Supernatants prepared by incubating Contortrix venom with fibrinogen, which cleaves the B peptide, were also chemotactic, whereas no activity was present in supernatants from Arvin venom, which cleaves peptides A, AP and AY. Thus fibrinopeptide B was chemotactic for the monocyte in addition to the neutrophil, as previously reported.

Monocyte chemotactic activity was also generated by the action of plasmin on human fibrinogen and shown to be associated with the D and E fragments but not with a mixture of fragments X and Y. When plasmin digestion was stopped at time intervals up to 24 h, monocyte chemotactic activity corresponded with the appearance of the D and E fragments. The monocyte chemotactic activity, contained in a 24 h digest, eluted from Sephadex G‐75 at V₀, corresponding to the expected position of the D and E fragments whereas neutrophil chemotactic activity eluted with molecules of molecular size of approximately 30 000 daltons. Thus fragments D and E derived from plasmin digestion of fibrinogen attract the monocyte whereas only the small uncharacterized peptides were chemotactic for the neutrophil. These different profiles of chemotactic activity for the neutrophil and the monocyte in terms of plasmin digestion products of fibrinogen may be of significance in the events leading to the accumulation of these cells in vivo during fibrin deposition.