Hemolymph characteristics were examined in experimentally cultured blue mussels Mytilus galloprovincialis Lmk, from 2 locations in the Ria d e Vigo (NW Spain). The highest protein, lysozyrne and agglutinin values were found in July. The increase of agranular hemocytes and the decrease of granular blood cells in April was associated with parasitism by Mytilicola intestinalis. No linkage was found between the number of circulating hemocytes and hemolymph component (lysozyrne, agglutinins and proteins) concentrations with parasitism. It is suggested that the number of circulating hemocytes, lysozyme and proteins levels and agglutination titer were related to the reproductive cycle and varied according to the location of mussels.