Studies on membrane receptors have been performed on the Nereis coelomocytes using various lectins. In the agglutination assay, only LCA and WGA appeared nonreactive. Fluorescent lectins showed the poor reactivity of the eleocytes and the diversity of the receptors according to the granulocyte types. Types I-granulocytes reacted only with Con A. Type II-granulocyte membrane contained mannose and galactose receptors (reactivity with Con A, PNA and SBA). The type III-granulocyte membrane revealed the presence of mannose and fucose receptors (UEA, AAA). Electron microscope investigations with HRP-DAB or mannosyl labelled Con A, RCAI and LTA have confirmed the distribution of the membrane receptors.