Application of antibacterial drugs plays an important role in the control of bacterial diseases in intensive fish culture. However, since the scientific basis for the use of these drugs is yet inadequate, there is a growing concern about the safety of fish medication for the consumer, the environment and the target animals. This led to the research presented in this thesis, which is focused on the influence of the antibacterial drug flumequine on the defence system of European eel (Anguilla anguilla L., 1758). Eel appeared to be an interesting model because the initial, pharmacokinetic investigations demonstrated that flumequine is very slowly eliminated from both plasma and tissues in this species. Treatment of eels with flumequine resulted in an enhanced proliferation of (probably surface immunoglobulin negative) lymphoid cells as assessed by means of an in vivo lymphocyte stimulation assay and monoclonal antibodies to eel immunoglobulin, respectively. The effect of the drug on the integral functioning of the defence system was subsequently examined by using a ,challenge with the parasitic nematode Anguillicola crassus. The resulting parasite recoveries pointed to an improved protection in flumequine-treated eels, which seemed to be related to a modulation of the cellular response.