In this study an immunofluorescence (IF) method was used to investigate the antigens of viruses and atypical bacteria in respiratory tract infections (RTI) in pediatric and adult age groups. In this prospective study of 2 years (1998 - 2000), IF was used to investigate the antigens of 7 viral and 3 atypical bacteria to be used for the etiological diagnosis of RTI. Sputum (33.6%) and nasopharyngeal aspirate specimens were obtained from pediatric patients (Group I, 76 cases) and adults (Group II, 135 cases) with RTI symptoms. Antigen detection rates were found to be 44.7% in Group I and 67.4% in Group II (P < 0.05). The following rates for specific antigens in Groups I and II, respectively, were as follows: Chlamydia pneumoniae, 17.1 and 13.3% (P > 0.05); Mycoplasma pneumoniae, 0 and 9.6% (P < 0.05); influenza A virus, 3.9 and 16.3% (P < 0.05); adenovirus, 3.9 and 14.8% (P < 0.05); parainfluenza virus type 1, 5.3 and 7.4% (P > 0.05); respiratory syncytial virus, 9.2 and 1.5% (P < 0.05); parainfluenza virus type 2, 3.9 and 3% (P > 0.05); and influenza B virus, 1.3 and 1.5% (P > 0.05). Mixed agents were found at a rate of 2.6 and 3.7% (P > 0.05) in Groups I and II, respectively. Parainfluenza virus type 3 and Legionella pneumophila antigens were not found. Since detecting etiological agents provides an important guide for determining the most appropriate antibiotic therapy, this IF method could be applied in clinical practice for arriving at a correct diagnosis and administration of effective treatment.