Abstract Hemolymph from the Pacific oyster ( Crassostrea gigas) contains lectins that agglutinate horse (Gigalin E) and human (Gigalin H) erythrocytes. The gigalins also agglutinate bacteria, including Vibrio anguillarum, and were adsorbed from oyster hemolymph at different temperatures by living, heat-killed, and freeze-dried V. anguillarum cells. Baseline activities of the two gigalins were established by measuring their activities in oyster hemolymph over a period of 4 years. A normal distribution of Gigalin H activity (mean titer 139) was found, whereas the distribution of Gigalin E activity in the same samples was skew (mean titer 512). No covariance was observed between the two agglutinin activities. Increased lectin activity above this baseline was found in oysters exposed for varying time intervals to V. anguillarum at different seasons and temperatures over a period of 2 years. Such exposure resulted in an increase in activity (titer) of four- to nine-fold for Gigalin E and three- to seven-fold for Gigalin H when compared with controls, and in augmentation in the hemolymph of a protein with the same electrophoretic mobility as affinity-purified oyster lectins (gigalins). Challenge with either living or heat-killed bacteria resulted in a significant increase of Gigalin E activity, whereas results for Gigalin H were variable. Oysters challenged with bacteria were observed to filter normally with open shells during the experiments. Also, no increase was found in hemolymph calcium that could indicate anoxia following bacterial challenge (0.49 ± 0.004 mg mL -1) compared to unexposed oysters (0.50 ± 0.001 mg mL -1). Increase in the concentration of free amino acids in oyster hemolymph was observed following exposure to bacteria (15.05 mM) and anaerobiosis (13.51 mM) compared to controls (9.06 mM), and changes (in mol %) of individual amino acids differed considerably between hemolymph from animals challenged with bacteria and animals kept anaerobic. The augmented lectin activity in oyster hemolymph, following in vivo exposure to increased bacteria in the seawater, suggests their involvement in enhancing bacterial clearance and defense in the oyster.