Bivalve molluscs, as filter-feeding organisms, are known to accumulate metals that can produce deleterious effects on organisms. The phagocytic activity of haemocytes and lysosomal alterations in the digestive gland cells were measured in the freshwater Asian clam exposed to cadmium, in order to assess the possible use of immunocompetence and lysosomal responses as biomarkers of freshwater quality. Clams were exposed in the laboratory to nominal concentrations of 3, 10, 21.4, 46.5 and 100 microg 1(-1) of cadmium and sampled after 7, 15 and 30 days of exposure. The results show a decrease of phagocytic activity after only 7 days of exposure to 10 microg 1(-1) of cadmium. This response was also observed as the exposure time was increased. Lysosomes in the digestive cells increased in size and number after 7 days of exposure as cadmium concentration increased. After 30 days of exposure, a decrease in size and number indicated a change in the response to the metal from concentrations of 46.5 microg 1(-1) of cadmium. A dose and time response both in phagocytic activity of haemocytes and lysosomal structure demonstrated a possible use of these biomarkers in freshwater biomonitoring.