This study assessed the effects of mechanical agitation, hypo-saline conditions, and exposure to the air on the Akoya pearl oyster, Pinctada imbricata, focusing specifically on the immunological activity of haemocytes. Both phagocytosis and phenoloxidase activity decreased significantly when oysters were exposed to all three stressors. Transient decreases were also evident in total haemocyte counts after mechanical stress and exposure to air, while significant increases in total haemocyte counts were evident after exposure to low salinity. Acid phosphatase activity increased significantly when oysters were exposed to air. The frequency of granulocytes in the haemolymph increased significantly when oysters were stressed by hypo-saline conditions, whilst the relative frequency of granulocytes did not differ significantly after mechanical agitation or exposure to air. The total protein content of haemolymph increased significantly when oysters were stressed by mechanical agitation and low salinity. These results suggest that fluctuations in environmental conditions affect circulating haemocytes and their cytochemistry, and that the different immunological parameters tested were influenced uniquely according to the type of stressor.