INTRODUCTION Swine influenza (SI) is an important viral disease caused by influenza A viruses that belong to the family Orthomyxoviridae. Influenza A viruses (IAV) are characterised by subtype of the two major surface glycoproteins, haemagglutinin and neuraminidase. The natural reservoir for IAVs are waterfowls, but both human and swine are also considered as its hosts. Swine has been considered as a „mixing vessel“ of new IAVs because of its susceptibility to infection with IAVs of different host origin. This doctoral thesis reports on a serological and virological survey for swine influenza virus (SIV) in pig population in Croatia. METHODS A total of 1536 blood sera samples were investigated for presence of SIV antibodies and 410 sera samples were examined for presence of HA antibodies by haemagglutination inhibition (HI) assay. 142 lung samples and 29 nasal swabs were investigated by molecular methods of SIV detection, with generic primers capable of detecting M gene which is common to all influenza A viruses. Due to subtyping od SIV, amplification of HA and NA gene fragments was performed and phylogenetic analysis was done. Detection of SIV antigen was made by IHC in lung samples collected post mortem. RESULTS In total of 1536 examined blood sera samples by ELISA, antibodies for SIV nucleoprotein (NP) were detected in 466 (30.3%). Results made by HI indicate that the most common subtype of SIV in Croatian pig population was H1N1 (44.6%), followed by H3N2 (42.7%) and H1N2 (26.3%). Antibodies against at least one subtype were detected in 62.19% of the blood sera samples. SIV antigen was detected in 8 of 28 lung tissue samples. M gene was detected in 9 of 142 lung tissue samples and in 7 of 29 nasopharyngeal swab samples. Phylogenetic analysis of amplified HA and NA gene fragments in 9 Croatian isolates indicates the presence of subtype H1N1 virus of wholly avian origin. IX CONCLUSIONS Different H1N1, H1N2 and H3N2 swine influenza viruses circulate in Europe. Some of those viruses are widespread but there is also a regional variability. There results demonstrated that subtypes H1N1, H1N1 and H3N2 are circulating in pig population in Croatia. Pandemic strain H1N1pdm which is circulating in some European countries is still not detected in Croatia. The continuous monitoring, surveillance and molecular characterization of the circulating SIVs is of major concern. Understanding the role of swine in spreading of these circulating viruses is necessary.