The study of hemogram characteristics in a population of cultured mussels through a period of 18 months revealed that the concentration of circulating Mytilus galloprovincialis hemocytes varied, following a seasonal pattern, being positively correlated with water temperature. Most mussels had between 1 and 5 x 10(6) hemocytes/ml of hemolymph. Differential counts showed that M. galloprovincialis hemolymph contained more granulocytes than hyalinocytes. The hemolymph cell concentration was not associated with size or weight of mussels. Circulating hemocyte concentration did not decrease in mussels with reabsorbing gonad, despite the hemocytic accumulation inside and around gonad follicles. No significant difference in hemolymph cell concentration was found between two groups of mussels of different ages. Nevertheless, older mussels had higher mean values of circulating granulocytes although these differences were only significant in the winter sample. Infestation by the protistan Marteilia refringens caused a significant increase of hemocytes in the hemolymph. In addition, the occurrence of some parasites induced significant changes in the proportion of circulating hemocyte types. Results suggested that a hemocytic response was detected in hemolymph when the foreign organisms are pathogenic and evoke a heavy inflammatory reaction. Mussels with large or numerous granulocytomas had a higher number of circulating granulocytes and hyalinocytes than those without granulocytomas. Copyright 1998 Academic Press.