Japanese flounder (Paralichthys olivaceus) head kidney (HK) leukocytes were incubated with $10^3$ to $10^{-3}$ ng levamisole/ml for 4, 24 or 48 h and then assayed for their natural cytotoxic activity against xenogeneic tumor cells. This activity was slightly increased after 24 h of incubation. In a second experiment, fish were fed 0, 75, 150 or 300 mg levamisole/kg diet for 10 consecutive days. The fish were then fed a commercial non-supplemented diet and sampled 0, 1, 2, 3, 4 or 6 weeks post-administration of levamisole. The cytotoxic activity was found to be increased with increasing levamisole dose and remained greatly enhanced until the end of the experiment. In conclusion, levamisole enhanced flounder natural cytotoxic cell activity both in vitro and in vivo and had a great lasting action when administered by feeding.