The aim of the study was to determine the activity of four beta-lactam antibiotics against nosocomial strains of gram-negative bacilli. Two antibiotics combined with beta-lactamase inhibitors: timentin (TIC/CLAV) and tazocin (PIP/TZB) and two carbapenems: imipenem and meropenem were applied. The clinical strains were isolated from patients hospitalized in the following wards: surgery and intensive care unit of State Clinical Hospital No 1 in Warsaw. The strains were identified in the automatic ATB system using ID 32 E and ID 32 GN strips. The susceptibility of isolates to antibacterial agents was determined in the automatic ATB system using ATB G- and ATB PSE strips. The susceptibility of the strains to imipenem, meropenem, timentin and tazocin was tested by disc-diffusion method. 157 strains of gram-negative bacilli were cultured. 100 strains were isolated from patients hospitalized in surgical ward and 57 strains from patients hospitalized in ICU. Nonfermenting rods dominated among isolated strains-91. The results obtained indicate that multiresistant gram-negative rods causing serious therapeutic problems are often isolated from clinical specimens. The contribution of nonfermenting rods, especially Pseudomonas spp. and Acinetobacter spp. to the etiology of infections in hospitalized patients has increased. Infections caused by these strains are difficult to cure. Tazocin and carbapenems (imipenem and meropenem) are highly active in vitro against the examined strains of gram-negative bacilli.