Cell surface properties of 78 strains of Acinetobacter baumannii of different origin (lower respiratory tract, wound, blood and environment) were investigated. The bacterial adhesion to collagen, fibronectin, fibrinogen and vitronectin was detected by particle agglutination assays. Salt aggregation tests were used to determine the cell surface hydrophobicity of isolated A. baumannii strains. We found that A. baumannii strains originating from patients with wound infection and bacteraemia showed significantly lower aggregative properties compared to respiratory and environmental strains. Electron microscopic investigations revealed more fimbriated bacterial cells among the highly aggregative A. baumannii strains. This study demonstrates that the investigated A. baumannii strains can be divided into two different groups according to their cell surface properties and source of isolation, whereas the majority of strains, from the lower respiratory tract and the hospital environment expressed strong adhesive properties.