Myosin and fibrin as substrates of plasmin. Fibrin (prepared from 3 μM fibrinogen) was digested with plasmin (10 nM) in the absence or in the presence of 2 μM myosin under the conditions described in “Materials and methods” for the turbidimetric assay. The reactions were stopped at the indicated times by dissolving the clots in 100 mM Tris-HCl, pH 8.2, buffer containing 100 mM NaCl, 8 M urea, and 2% sodium dodecyl sulfate and boiling. The samples were analyzed with electrophoresis on 10% polyacrylamide gel under nonreducing conditions.