The present invention provides compositions exhibiting in vivo activity of fibrin in an in vitro setting, in vitro assays comprising such compositions, methods of producing such compositions, and methods of using such compositions and assays. The compositions of the invention include molecules with the biochemical properties of 1) high affinity binding to fibrin receptors and 2) activation of cell-signaling systems comparable to that observed in vivo by fibrin. The fibrin compositions of the invention are compatible both in biochemical assays and cell-based assays, and thus useful for in vitro assays for screening of test agents that modulate cell activation and/or signaling pathways mediated by fibrin or associated with fibrin activity.