The fixed phagocytes are noncirculating cells in the digestive gland (hepatopancreas) of the American lobster (Homarus americanus) and other decapod crustaceans. They are attached to the outer walls of the terminal hepatic arterioles and lie in the hemal spaces, where they are bathed in blood; they remove foreign particles from the blood as it circulates through the digestive gland. This study has developed and adapted methods for the in vivo assessment of the activity of the fixed phagocytes of lobsters by measuring the uptake of foreign particles from the blood. After fluorescent microspheres were injected into the blood, samples of the digestive gland were excised, arterioles were prepared for microscopy, confocal laser scanning micrographs were collected and microspheres and cells were counted. These methods may be used in larger-scale studies to assess the state of activity of this important part of the immune system of lobsters, and to compare immune activity in lobsters exposed to various environmental and anthropogenic stresses.