In mammals, estrogens have both dose- and cell-type specific effects on immune cells and may act as pro-inflammatory and anti-inflammatory stimuli depending on the setting. Evidence has been provided that in the bivalve mollusc Mytilus the natural estrogen 17-estradiol (E₂) can affect neuro-immune functions. In this work the immunomodulatory role of E₂ in Mytilus hemocytes, the cells responsible for the innate immune response, was investigated. E₂, in a narrow concentration range (5-25 nM), rapidly stimulated phagocytosis and oxyradical production in vitro; higher concentrations inhibited phagocytosis. E₂-induced oxidative burst was prevented by the nitric oxide synthase inhibitor L-NMMA and by SOD, indicating the involvement of both NO and O₂^-; NO production was confirmed by nitrite accumulation. The effects of E₂ were prevented by the antiestrogen Tamoxifen and by specific kinase inhibitors, indicating a receptor-mediated mechanism and involvement of p38 MAPK and PKC. E₂ induced rapid and transient increases in the phosphorylation state of PKC, as well as of a CREB-like transcription factor, as indicated by WB with specific anti-phospho-antibodies. Localization of ER- and ER-like proteins in hemocytes was investigated by immunofluorescence confocal microscopy. The effects of E₂ on the immune function were also investigated in vivo, at longer exposure times (6 and 24 hrs) in E₂-injected mussels. E₂ significantly affected hemocyte lysosomal membrane stability, phagocytosis and extracellular release of hydrolytic enzymes: lower concentrations resulted in immunostimulation, whereas higher concentrations were inhibitory. Overall, the obtained data indicate that the physiological role of E₂ in immunomodulation is conserved from invertebrates to mammals.