Occlusive microvascular thrombi are prominent in LLC footpad tumors derived from plasminogen-deficient mice. Electron microscopic evaluation of footpad tumor vasculature revealed widespread microvascular thrombi within tumors derived from Plg^– mice (A-C) but not tumors collected from control (D), Fib^– (E), or Plg^–/Fib^– (F) animals 14 days after initial tumor cell transplantation. These representative sections from Plg^– mice include examples of vessels with small platelet aggregates (A) and vessels with advanced, highly occlusive thrombi (B-C). Occlusive platelet thrombi (arrowheads) were found in areas populated with viable tumor cells (*) and where the endothelium appeared essentially intact (arrows). Note that occlusive intravascular thrombi were not a feature of the rapidly growing tumors of control mice and Fib^– mice. More significantly, no such platelet aggregates were observed in plasminogen-deficient mice lacking fibrinogen (F), animals that exhibited no impediment in tumor growth. Horizontal bars in each micrograph indicate 8 μm.