This study examined whether N-acetylcysteine (NAC), a glutathione precursor, could attenuate toxic effects of three fish anti-parasitic agents, trichlorfon, hydrogen peroxide ($H_2O_2$) and formalin, all of which are known to exert side effects through free radical production. Common carp Cyprinus carpio were fed with NAC (approx. 50 mg/kg/day) for 3 consecutive days prior to anti-parasite bathing for a 24 hr period. Mortality rates were examined during this 24 hr bathing period, and selected hematological and biochemical parameters were also assessed at the termination of anti-parasite exposure. The mortality rates and plasma glucose elevations caused by all three anti-parasitics were significantly reduced by NAC pretreatment. Trichlorfon, but not $H_2O_2$ or formalin, elevated plasma levels of aspartatetransaminase (AST) and alanine-transaminase (ALT), and these elevations were attenuated by NAC. There was no change in hematocrit values in any treatment. The results provide evidence for the attenuating effects of NAC against toxicity caused by anti-parasite agents that act through free radical-producing properties. The results found in this study also suggest that NAC may be administered to fish to minimize toxicity in fish parasiticide use.