In a comparative study on the efficacy of two surgical treatment procedures, 90 patients diagnosed with chronic maxillary sinusitis received antimicrobial prophylaxis. Maxillary sinus fluid samples were taken from these 90 patients before the initiation of antimicrobial prophylaxis, and were cultured for aerobic and anaerobic bacteria. In 81% of all samples, bacterial growth was present. Anaerobic bacteria were present in 29% of the cases (5% pure anaerobic and 24% mixed aerobic and anaerobic). Predominant aerobic bacteria were staphylococci, in 42% of all samples (mainly coagulase-negative), and streptococci (32%). Predominant anaerobic bacteria were Propionibacterium species (18%) and Gram-negative rods (9%). The antimicrobial susceptibility was determined for 20 antibiotics, and the beta-lactamase production was investigated. The prevalence of beta-lactamase-producing bacteria was unexpectedly high (isolated from 36% of all patients). The findings in this study indicate that not all antimicrobial agents used against upper respiratory tract infections are suitable for treatment of this group of patients when the susceptibility of bacteria present in sinus fluid is taken into account. The high prevalence of beta-lactamase-producing bacteria could influence the course of the disease, due to the breakdown of antibiotics by such strains. Recommendations are given for criteria to use when choosing antimicrobial agents to treat bacterial infections it) patients with chronic maxillary sinusitis.