The present study investigated the influence of polyunsaturated fatty acids (PUFA) on the immune system of germ-free piglets. Oil with increased content of omega-3 PUFA was administered to piglets from the experimental group (EG) for four weeks. Piglets from the control group (CG) received identical volumes of saline solution. At the age of 21 days both groups of germ-free piglets were inoculated perorally with Lactobacillus casei subsp. casei at a dose of 2 ml (1x10(8) mli). At the age of 28 days, i.e. after one-week colonisation of germ-free piglets with Lactobacillus casei subsp. casei, significant differences were recorded in phagocytic activity of neutrophils (PANe) and phagocytic activity of potentially phagocytizing cells (PA) (P < 0.05). Between EG and CG there have been observed no significant differences in absolute numbers of CD4+ and CD8+ T lymphocytes and numbers of IgM cells and in additional investigated parameters - number of CD2+ T lymphocytes, index of phagocytic activity of neutrophils (IPANe) and index of phagocytic activity (IPA). The total number of leukocytes (Le) in EG was also higher. Of the parameters determined in blood serum we observed a significant increase in concentration of alpha linolenic, eicosapentaenoic and docosahexaenoic acids and a parallel decrease in the level of arachidonic acid.