Lower respiratory tract infections [LRTIS] continue to be a major cause of morbidity and mortality worldwide especially in developing countries. Mycoplasma pneumoniae [M.pneumoniae] is an important cause of LRTIS. Imperical treatment of M.pneumoniae infection is getting ineffective because of the lack of a typical bacterial cell wall rendering the organism insensitive to cell wall active antimicrobials such as penicillins and cephalosporins. Because of this, the recovery of the organism from clinical specimens has significant therapeutic implications. However, the current way of diagnosis namely culture is complex, time consuming and relatively insensitive. Hence, the idea of the present investigation was to determine the causative bacteria of LRTIS with special emphasis on the role of M.pneumoniae and to evaluate a direct antigen detection technique [pneumofast Ag test] in comparison to the conventional culture technique for diagnosis of M.pneumoniae infection. The study included 132 patients clinically diagnosed as having LRTIS attended the Chest Department of Main Universty Hospital of Alexandria. Sputum samples were collected from all patients and divided into three portions. The first portion was used for Gram stain and for cultivation on blood, chocolate and MacConkey agar plates. The second portion was inoculated into mycoplasma diphasic medium, while the third portion was used for detection of M.pneumoniae antigen using enzyme immunoassay. The results revealed that, only110 cases were laboratory confirmed as having bacterial LRTIS. Males represented 60.91% while females were39.09%. The majority of cases [47.27%] had pneumonia .The most frequent isolated bacteria was S.pneumoniae [33.64%] followed by S.pyogenes [18.18%]. M. pneumoniae was the third encountered pathogen with an infection rate of 15.45%. The majority of M. pneumoniae positive cases were diagnosed significantly in the age group 10-< 30 years represented 14 [29.79%] cases from the total studied cases in this age group. The infection rate was slightly higher in males [16.42%] than in females [13.95%]. Pneumonia was the commonest clinical presentation of infection by the organism represented 13 [25%] of all pneumonic patients. Culture was considered as the gold standard in the diagnosis of M.pneumoniae infection. All 12 [10.91%] M.pneumoniae culture positive cases were also positive by Ag-EIA while another 5 [4.55%] cases were positive by Ag-EIA but negative by culture. This gave a sensitivity of 100%of Ag-EIA and 95.5% agreement between the results of both methods. Hence, it could be concluded that: the most common isolated pathogen from patients with bacterial LRTIS is S.pneumoniae. M.pneumoniae has an appreciable role as a cause of LRTI... M.pneumoniae isolation by culture method is complex time consuming and relatively of low sensitivity. Ag-EIA is a rapid, highly sensitive and specific method for diagnosis of M.pneumoniae in sputum and can substitute the conventional culture method.