Environmental pollution is a growing concern and, more importantly, pollution of the aquatic ecosystem is alarming. Marine pollution may be one of the reasons for disease incidence in marine organisms, which is caused due to adverse effects of pollutants on the immune system. Bivalves are commonly used as bio-indicators of marine pollution, and immunomodulation due to toxicants is one of the important bio-markers used. Perna viridis too have been used as a bio-indicator, but this study is, to our knowledge, a first report on immunomodulation produced by metals, in P. viridis. Animals were exposed to copper and mercury at their sub-lethal concentrations of 20 microg L(-1) and 10 microg L(-1), respectively. Immune parameters including phenoloxidase, reactive oxygen species generation, and phagocytosis were monitored. The study period was for 25 days (chronic long-term exposure) and objectives established whether metals produced immunomodulation and to understand the effects of long-term exposure on immunomodulation. Results showed that both metals adversely affected immune parameters studied and, interestingly, there appears to be some level of recovery (depuration) from the toxic effects of metals.