Holothuria poliibrown bodies are associated with the defence mechanisms of the host. Naturally and experimentally induced brown bodies are histologically constituted by nodules in a granular extracellular matrix containing distinct spherule cell types. Nodules of natural brown bodies consist in flattened amoebocytes surrounding large antigens, whereas nodules from experimentally induced brown bodies originate from the aggregation of phagocyting amoebocytes. An extracellular granular matrix characterizes both types of brown bodies. It probably originates from degranulation of spherule cells.