Objectives: Acinetobacter baumannii is one of the main causers of opportunistic nosocomial infections. Regarding its ability of rapid dissemination in hospital enviorment and acquiring mechanisms of antimicrobial resistance, it represents global healthcare-associated problem. Its virulence factors and resistance mehanisms, due to which it can rapidly colonise and infect patients, are still underestimated and poorly understood. The aim of this study was to evaluate in vitro effect of subminimal inhibitory concentrations of imipenem, ampicilin/sulbactam, azithromycin, rifampicin and colistin onto the ability of biofilm formation and sensitivity to bactericidal activity of human serum of clinical A. baumannii isolates. Material and methods: From the collection of A. baumannii isolates, based on microtiter biofilm formation assay, two groups were formed: one group containing 34 A. baumannii biofilm positive, and another with 34 biofilm negative isolates.Serum bactericidal tests were performed and serum sensitivity was detected by colony count after exposure to undiluted human serum for two hours. Serum sensitive isolates were the ones with viabile cell count <10% in comparison to control. MICs for imipenem, ampicilin/sulbactam, azithromycin, rifampicin and colistin were detected and after 18-24 hour exposition of isolates to antibiotic concentrations ½, ¼, ⅛ i 1/16 of MIC, the microtiter biofilm formation assay and serume bactericidal tests again conducted to determine effect of antibiotic subMICs onto these two virulence determinants. Results: Sixty-five of sixty-eight tested strains accounted for resistant to human serum. All tested imipenem and azithtromycin subMICs exhibited statistically significant suppression of biofilm formation in biofilm positive strains. The effect of rifampicin was statistically significant in biofilm formation at ½ and ¼ of MIC, as well as for colistin at ½ and ⅛ of MIC. Statistically significant effect of sensitivity to human serum bactericidal effect was detected for ampicilin/sulbactam and azithromycin at all four tested concentrations bellow MIC. Conclusion: Azithromycin exhibited the best effect among tested antibiotics. It suppressed biofilm formation and had serum bactericidal activity enhanced. Biofilm formation was impaired when medium containing ampicilin/sulbactam was used. Nontheless, rifampicin, imipenem and colistin also exhibited significantly different biofilm formation production suppression. Ampicilin/sulbactam also established effect on serum resistance. Ability to induce biofilm formation was not detected.