Haemopathological changes attributed to heavy metal poisoning observed in blood smears of Liza macrolepis (Smith) taken after exposures of 96 h to graded doses (mg/l) of copper (0.11-1.80), lead (1.15-18.36), and mercury (0.04-0.59), in a flow-through marine bioassay system. In general, changes in leucocytic profile appear to be correlated with pathological changes caused by increasing copper and mercury concentrations. By contrast, blood samples of mullets exposed to lead, showed significant polychromasia and +1 anisocytosis regardless of concentrations. The RBC count, haemoglobin content, and haematocrit percentages were less valuable in diagnosis of copper and mercury effects. These manifestations of poisoning by trace elements bear a resemblance to the pathological changes that have been shown clinically and experimentally in mammals. Consequently, blood measurements on marine organisms may be diagnostic of undesirably high levels of copper and mercury, and so may constitute useful indicators of marine pollution.