Generation of superoxide anion by stimulated haemocytes of Mytilus edulis was demonstrated using dihydrorhodamine 123 and quantified using reduction of nitroblue tetrazolium (NBT). In the presence of zymosan or phorbol myristate acetate, there was an increased reduction of NBT to formazan. The addition of superoxide dismutase (SOD) and iodoacetamide to the incubation medium resulted in a significant reduction in deposition of reduced formazan. Incubation of haemocytes with the SOD inhibitor diethyldithiocarbamate (DDC) gave rise to a small but significant increase in NBT reduction. The production of hydrogen peroxide by haemocytes was quantified using horseradish peroxidase-dependent oxidation of phenol red. The presence of SOD in the incubation medium together with zymosan resulted in a significant increase in H2O2 production. Haemocytes incubated with DDC prior to the assay or with sodium nitroprusside during the assay showed a decrease in H2O2 production with increasing concentration of the inhibitor.