The growing popularity of nanotechnology in the past decade has increased nanomaterial concentrations in the environment and the risk of their toxicity for aquatic organisms. Metal nanoparticles, which are easily absorbed and accumulated by fish, are probably able to interact directly with their immunocompetent cells. The objective of this study was to evaluate the in vitro effect of commercially available silver, gold and copper nanocolloids on the rainbow trout leukocyte and splenocyte activity. At high concentrations, all of the nanocolloids studied had adverse effects on the proliferative response of trout lymphocytes, and the most toxic of them, silver, decreased also the respiratory burst activity of splenocytes. Low concentrations of silver nanocolloid, however, had a stimulating effect on the lymhocyte proliferation.