The aim of this paper was to analyse aerobic and anaerobic bacterial flora colonizing pleural drains in 32 patients with lung tumor undergoing pulmonary resection and receiving antimicrobial prophylaxis. Fluid from pleural drains was taken up on the day of operation and on the 3-rd or 4-th day after the operation. Significant number of aerobic bacteria, mainly methicillin-resistant coagulase-negative staphylococci and multidrug resistant Gram-negative rods, were isolated from 30 (46.9%) specimens. Lower number of anaerobic bacteria were found in 24 (37.5%) specimens. Colonization of the pleural drains does not mean infection, (in only one patient isolated Pseudomonas aeruginosa could be the etiologic agent of infection) however knowledge about bacterial species found in drain fluid in a local population and drug susceptibility of isolated strains allows to propose effective antimicrobial prophylaxis.