Among 1,272 H. influenzae isolates obtained from patients with lower respiratory tract infections, 5.6% were found to be encapsulated. The most common were serotypes b (40.3%) and e (38.9%) followed by serotypes f (16.7%) and d (4.1%). All encapsulated isolates within a particular serotype showed high genetic homogeneity and did not produce ␤-lactamases.Haemophilus influenzae is responsible for respiratory tract infections as well as severe invasive infections such as meningitis, bacteremia-sepsis, epiglottitis, and septic arthritis. Although the overwhelming majority of invasive Haemophilus sp. infections are caused by H. influenzae serotype b, other serotypes as well as nontypeable isolates (NTI) have also been associated with invasive diseases (2,3,6,12,13,17,18,24). The aim of this study was to investigate the prevalence and serotype distribution of encapsulated H. influenzae isolated from patients with lower respiratory tract infections (LRTIs).From 1996 through 2001, 1,272 H. influenzae isolates, representing the whole country, were collected during a surveillance study in 25 centers in Poland. The study included outpatients and hospitalized patients (sampled within 48 h of admission) with clinically proven community-acquired LRTIs (exacerbations of chronic obstructive pulmonary disease or pneumonia) from whom samples of bronchoalveolar lavage, blood, or sputum (evaluated with respect to the numbers of white blood cells and epithelial cells and the presence of the mucus threads) were taken (1). Although in local laboratories all isolates were identified by standard procedures, 4.07% of isolates were misidentified. PCRs were run to confirm species identification and to detect capsule-and serotype-specific genes. Supernatants obtained from boiled colonies of H. influenzae were used for duplex PCR for species identification and capsule gene detection (4, 5). For isolates for which the specific products for capsule genes were obtained, the PCRs for serotypes a to f determination were run with primers a1-a2, b1-b2, c1-c2, d1-d2, e1-e2, f1-f2, respectively (4). Quality control strains of H. influenzae of serotypes a to f used for PCR were provided by Oxford Public Health Laboratory (John Radcliffe Hospital). The H. parainfluenzae strain used as the negative control (strain no. 2259/96) was derived from our collection.The production of ␤-lactamases was determined by a nitrocefin assay according to the instructions of the manufacturer (bio Merieux, Marcy l'Etoile, France). The relatedness among encapsulated isolates was evaluated by restriction fragment length polymorphism of SmaI-digested chromosomal DNA analysis, using pulsed-field gel electrophoresis (PFGE), and interpreted as previously described (21,22).A total of 72 (5.6%) H. influenzae isolates were found to be encapsulated. The most common were H. influenzae serotypes b (40.3%; n ϭ 29) and e (38.9%; n ϭ 28) followed by serotypes f (16.7%; n ϭ 12) and d (4.1%; n ϭ 3). The results of comparisons of the frequencies of recovery of different serotypes and NTI s...