Hemocytes were sampled from C. gigas adult oysters under 24 experimental conditions (total of 144 oysters): oysters of three geographic origins (6 oysters per condition; Atlantic coast-La Tremblade, Normandie-Bay des Veys and Mediterranean Sea-Thau lagoon) were exposed to four kinds of bacterial challenges and hemolymph were sampled and pooled at two post challenge times. The bacterial challenges performed by oyster immersion were (1) alive non virulent Micrococcus luteus and Vibrio tasmaniensis (2.5 × 108 bacteria/L for each strain), (2) alive virulent V. splendidus (5 × 108 bacteria/L), (3) mix of heat killed virulent V. splendidus and V. aestuarianus (2.5 × 108 bacteria/L for each strain) and (4) unchallenged oysters. For each condition, hemolymph was collected at 12 and 24 hours after challenge from the pericardial cavity through the adductor muscle. After hemolymph collection, hemocytes were isolated by centrifugation to discard plasma (700 g for 10 min. at 4°C) for further RNA extraction. For RNA extraction of mantle tissue, oysters from the Mediterranean Sea (Thau lagoon) were harvested by dissection. All experimental infections were performed according to the Ifremer animal care guideline and policy.