The aim of the study was to assess the influence of norfloxacin nicotinate on the selected immune functions of the cells isolated from head kidney of rainbow trout (Oncorhynchus mykiss). To estimate the drug concentrations reached in the organ, fish were given norfloxacin nicotinate orally for five days or in a single intraperitoneal injection, and the drug residues in head kidneys were determined by HPLC. The highest concentration of norfloxacin, 25.60 µg/g, was found 24 h after the drug administration at 60 mg/kg in the single intraperitoneal injection. 7 days after application, the drug amounts in the tissue reached approximately 10% of the concentrations found 24 h after application, that is 1.41-2.38 µg/g. Then in the in vitro studies leucocytes isolated from fish head kidneys were tested on the lymphocyte proliferative response on mitogens and on respiratory burst activity of phagocytic cells after cell incubation with norfloxacin nicotinate at concentrations of 1, 5, 10, 20, 30, 40, 60 µg/ml. Suppressive effect of the drug was observed on B cell proliferation when the cells were exposed to the higher used concentrations, 30-60 µg/ml, T cells were less sensitive, the effect was seen only in the highest concentration. The drug caused stimulation of respiratory burst activity at 5 µg/ml. Other used concentrations had no effects on the studied parameter.