1. In vitro phagocytosis of marine bacteria by fresh oyster leucocytes, though readily demonstrable in most cases, was by no means an invariable phenomenon. When it occurred, it was frequently accompanied by a massive sticking of bacteria to the leucocytes. The flagellar portion of the bacterium might be so caught by the amebocyte that the bacterium was unable to escape, even though the body was not in contact with the amebocyte. 2. An irregular but repeated formation of an extracellular clot is described as seen in vitro by phase microscopy. Reasons for believing that it is a true phenomenon in the oyster are given. 3. Intravascular clotting or thrombosis was produced by the intracardiac injection of tissue extracts. The clotting disappeared spontaneously within two hours after the injection.