In the Eastern oyster (Crassostrea virginica) hemocytes are important effector cells for maintenance of defense against pathogenic microorganisms. Various forms of β-glucans have been suggested for use in shrimp and fish aquaculture because of their potential to enhance disease resistance via hemocyte activation. To gain insight to the effects of these compounds in molluscs, changes in circulating oyster hemocyte numbers, population profiles, and adhesion were quantified after injection of β-1, 3-glucans. These agents included zymosan A or MacroGard, an immune stimulant suggested for aquaculture applications. Results were compared with those from estuarine water-injected control oysters at daily intervals for 3 days. MacroGard produced a rapid (by 1 day) elevation of total circulating hemocytes, which remained significant for 3 days; zymosan A stimulation of this response was not statistically significant. MacroGard administration resulted in a sustained increase in percent granulocytes in circulation. Zymosan A also produced an increment in the numbers of granulocytes that became significant by 3 days. Granulocytes are considered to be the most immunologically active hemocyte subclass. Both β-glucan preparations promoted hemocyte aggregation at days 1–3 after injection. Analysis of avidity of hemocyte aggregation using an index of clump cellular density confirmed this observation. Hemocytic aggregation is thought to be an indicator of cell activation. These results suggest that β-glucans can act to increase the numbers of circulating hemocytes available for defense responses, to promote longterm changes in the hemocyte profile favoring immunologically active cells, and to enhance hemocytic aggregation. Further studies of the physiological consequences of these effects of β-glucans on oyster hemocyte defense parameters are in progress.