A significant mortality of the American lobster (Homarus americanus) in western Long Island Sound in 1999 seriously impacted the harvest of this species. Whereas the exact cause or causes of the mortality are still not precisely known, dead and dying lobsters in the initial phase of the die-off were diagnosed with infections with paramoeba, a newly recognized disease condition of lobsters. Because the immune system represents the first line of defense against disease-causing agents and is one of the most sensitive systems to environmental stressors, this study is aimed at the development of new assays to quantify the immune system of lobsters. Flow cytometry allowed the discrimination of different populations of hemocytes based on their relative size and complexity. Also using flow cytometry, natural killer (NK) cell-like activity and its stimulation by human recombinant interleukin 2 (IL-2) are described for the first time in lobsters, as is the expression of TLR2, a pattern recognition receptor, on granular hemocytes. Apoptosis is also measured for the first time in lobster hemocytes in higher proportion in nongranular hemocytes than in granular hemocytes. Also shown is the fact that circulating hemocytes fail to proliferate on stimulation, suggesting that they are terminally differentiated and originate from a separate hematopoietic organ. Overall, several new assays are developed to allow the quantitative evaluation of disease-relevant immune functions for future use in health assessment of the American lobster.