Panulirus homarus (Linnaeus, 1758) being a species of great commercial importance fetches high demand in the international market. The demand of live lobsters depends on the health status of the animals. Study on the defence mechanisms in P. homarus showed three cell types in the haemolymph: hyaline cells, semi granular cells and large granular cells. The haemocy1es are produced from the haematopoetic tissue located in the epigastric region, they are lobular with a size range of 30-875 jJm. The immune status of healthy lobsters showed that the total haemocy1e count was 8.7 ± 3.8 x 106 cell mr l and the phenoloxidase activity 30.61 ± 5.6 IU mg protein- 1 • Studies on physiological stress like moulting, eyestalk ablation showed variation in the immune parameters (P < 0.05) . Upon starvation the THe and PO activity was found to decrease drastically. Feed trials showed that mussels, the preferred diet of lobsters was found to show better immune response over those fed with fish diet. Environmental stress factors like salinity, pH, dissolved oxygen and ammonia showed significant effect on the immune system of P. homarus. Live transportation of lobsters by dry method was found to suppress the PO activity but no change was seen in THe. Injection studies with bacteria showed the lowest peak during 30 minutes incubation for THe whereas it showed highest peak for PO activity for the same incubation time. Both THe and PO levels came back to normal after 24 hours. On injection with laminarin an immunostimulant similar result was obtained. Farm made artificial feed supplemented with chitosan was found to enhance the activity of the immune system . Satisfactory result for immune enhancement was not observed with commercial immunostimulant supplement. Analysis of serum PO activity and total haemocy1e count can be used as indicators of the health in crustaceans, which can be monitored, even in farm conditions. Routine monitoring of health status is of great importance with regard to successful aquaculture practices.