Hemocytes from the mollusk Mytilus galloprovincialis Lmk. were maintained for more than 20 days in Leibovitz L-15 medium supplemented with salts and antibiotics. Survival of cells cultured in foetal calf serum (FCS)-supplemented L-15 medium was similar to that of those cultured in inactivated hemolymph-supplemented L-15 medium. Cell activation by lipopolysaccharide (LPS) induces an increase of the number of α-subunits of the interleukin-2 (IL-2) receptor (IL-2Rα) in the cell membrane. The best response in the expression of IL-2Rα subunit was obtained after 3 days of culture. In cultured cells, as in fresh hemocytes, LPS induced a higher expression of IL-2Rα subunit than that induced by IL-2.