The main aim of this study was to investigate the immune system of the European flat oyster, Ostrea edulis, to determine the suitability of using immunological indices as potential biomonitors of water quality, particularly in reference to petroleum hydrocarbons. Immunocompetence was inferred through a measure of total and differential haemocyte concentration, larger granulocyte motility and diameter change, haemolymph hydrogen peroxide concentration and haemolymph lysozyme activity. Before any of these indices could be used as biomonitors, a baseline investigation had to be conducted. The baseline survey indicated that there was a strong seasonal signal for immunosuppression at the beginning of both the major and the minor spawning seasons. This immunosuppression appeared to be the result of partitioning of energy into the reproductive process. Historical occurrence of the two major diseases of this species occurred within these times. A neutral red uptake assay was applied to the haemocytes of O. edulis to investigate the sensitivity of this novel assay as an immunological indicator. The assay proved sensitive, reliable and consistent and supported the previous findings of pre-spawning immunomodulation. Oysters were exposed to the water soluble fraction of crude oil. Increasing concentrations of petroleum hydrocarbons resulted in an increase in haemolymph hydrogen peroxide concentration, increased haemocyte concentration and decreased large granulocyte motility. Haemolymph lysozyme activity and large granulocyte diameter change appeared to be influenced more by seasonal or physiological factors than by the contaminants. The increase in concentration of large granulocytes was accompanied by a decrease in large granulocyte viability which was confirmed by a reduction in the concentration of the neutral red dye retained by the haemocytes. Comparisons were made with immunological indices, concentrations of petroleum hydrocarbons in the water soluble fraction and with hydrocarbons retained in the tissue.