The stimulatory effect of various fibrin preparations on plasminogen activation by tissue plasminogen activator, was studied by the Coa-set Fibrin Monomer test (Kabi). Fibrin obtained by complete conversion of purified fibrinogen demonstrated a greater stimulatory effect on plasminogen activation than did equal amounts of fibrin obtained by partial conversion of fibrinogen. Soluble fibrin generated by treating human plasma with minute amounts of thrombin or bathroxobin, resembled partially converted purified fibrinogen. The plasminogen activating effect of completely converted fibrinogen was similar in thrombin and bathroxobin incubated samples. In preparations of partially converted fibrinogen and in plasma samples, bathroxobin digested fibrinogen expressed a more pronounced stimulatory effect on plasminogen activation than did thrombin digested specimens. The underlying mechanism for these differences are discussed.