Abstract The studies compared opossum to human blood. 1. 1. Coagulation mechanisms appeared to be fundamentally similar although factors II, VII and X assayed well below and V, VIII, IX, XI and XII above the human range. Fibrinogen appeared to be quantitatively similar to human but different in its poor clottability with bovine thrombin and inability to clump staphylococci. 2. 2. Platelets were variable in number and size and adhered to glass and aggregated with ADP much as do human platelets. On the other hand, they aggregated poorly or not at all with collagen, epinephrine and thrombin. 3. 3. Platelet ultrastructure was similar to human except for two striking differences. Many platelets contained a circumscribed mass of wavy fine fibrillar material of unknown composition. Also, striated elongated granules were quite common. 4. 4. Erythrocytes were slightly smaller than human. Leukocytes were not very different and lymphocytes predominated. 5. 5. Serum albumin was very low by the automated dye-binding method but was much higher by electrophoresis. 6. 6. The serum enzyme values were very much higher than those seen in normal humans.