When streptokinase and highly purified human plasminogen are added to human serum or to partly purified or highly purified preparations containing the third component of complement (C'3), either rabbit or human, a chemotactic factor is generated. This chemotactic factor is a split product of C'3 and is dialyzable, fast moving electrophoretically, slowly sedimenting in sucrose density gradient ultracentrifugation, and has an approximate molecular weight of 6000. It is calculated that this fragment accounts for approximately 4% of the intact molecule. The C'3 fragment has the following biologic properties: It is chemotactic for rabbit PMN's in vitro, it causes accumulation of PMN's in vivo, and it increases vascular permeability in rat skin. In addition to generating a chemotactic factor, plasmin destroys the complement-associated chemotactic factor that is a trimolecular complex consisting of the fifth (C'5), sixth (C'6), and seventh (C'7) components of complement. This has been shown by a loss of chemotactic activity, as well as a dissociation of the C'5, C'6, C'7 complex and a destruction of C'6 hemolytic activity. The biologic significance of the plasmin-generated chemotactic factor is discussed in relation to other recently discovered biologically active fragments of C'3.