The study of tinctorial affinities, in vitro behaviour and ultrastructure of the hemocytes of Pinna nobilis allows us to group them into two main families: basophilic, neutrophilic and acidophilic granulocytes; hyalinocytes, three categories can be distinguished among these cells: small hyalinocytes, hyalinocytes containing smooth endoplasmic reticulum and the macrophage population. The hemocytes participate in the regulation of metabolism by the storage and degradation of glycogen. They contribute to the detoxification process by the sequestration of mineral elements in their lysosomes. Their role in antibacterial defense is suggested by their bacterian phagocytic activity.