Studies on lymphoid organ spheroid (LOS) cells of Penaeus rnonodon were undertaken. Phenoloxidase and peroxidase assays showed that LOS cells have characteristics simllar to semi-granular and, in particular, large granular haemocytes. The mean percentage of LOS cells positive for phenoloxidase and peroxidase was 85 + 23 and 82 * 23%, respectively. There was no significant difference between the sites of phenoloxidase and peroxidase activity in LOS cells ( t = 1.617, df = 29, p > 0.05). The relative sectional area occupied by LOS cells relative to that of the strornal matrix cells from both laboratory-held and farmed prawns was not correlated to increasing weight or total length of the prawns (p > 0.05). An apoptosis detection assay showed that LOS cells were often apoptotic whilst stromal matrix cells were not. There was a significant difference (t = -5.533, df = 58, p < 0.05) in the mean percentage of apoptotic spheroid cells between laboratory-held prawns (52 * 24 % ) and farmed prawns with rnidcrop mortality syndrome (MCMS) (80 i 12%). In conclusion, LOS cells have the chdracteristics of exocytosed, granular haemocytes that have phagocytosed foreign material, particularly viruses, and probably constitute a major mechanism for penaeid antiviral defense.