Bacteria isolated from respiratory tract infections were collected in cooperation with institutions located throughout Japan, since 1981, and the Ikemotor et al. have been investigating susceptibilities of the isolates of various antibacterial agents and antibiotics, and the relationships between the isolates and backgrounds of the patients and so forth each year. We discuss the results in detail. In 20 institutions around the entire Japan from October 1993 to September 1994, 584 strains of bacteria were isolated mainly from sputa of 473 patients with respiratory tract infections and presumed to be the etiological agents. MICs of various antibacterial agents and antibiotics were determined against 91 strains of Staphylococcus aureus, 98 strains of Streptococcus pneumoniae, 122 strains of Haemophilus influenzae, 91 strains of Pseudomonas aeruginosa (non-mucoid), 34 strains of Pseudomonas aeruginosa (mucoid), 42 strains of Moraxella subgenus Branhamella catarrhalis, 25 strains of Klebsiella pneumoniae etc., and the drug susceptibilities of these strains were measured except the strains which died during transportation. 1. S. aureus S. aureus strain sfor which MICs of methicillin was higher than 4 micrograms/ml (methicillin-resistant S. aureus) accounted for 56.0%, but this frequency of the drug resistant bacteria was lower than the previous year's 61.4%. Arbekacin and vancomycin showed the highest activities against MRSA and MIC80s were 1 microgram/ml. 2. S. pneumoniae Benzylpenicillin among the penicillins showed potent activities against S. pneumoniae. Cefuzonam, cefotaxime and cefmenoxime among the cephems showed excellent antimicrobial activities against S. pneumoniae. Imipenem; carbapenems, showed the most potent activity, and MIC90 was 0.063 microgram/ml. 3. H. influenzae All the drugs tested were quite active against H. influenzae. Cefotaxime, cefmenoxime, cefuzonam and cefixime among the cephems showed the most potent activities, and MIC90 were 0.063 microgram/ml against H. influenzae. Ofloxacin also showed MIC90 of 0.063 microgram/ml. 4. P. aeruginosa (mucoid) Tobramycin showed the most potent activity against P. aeruginosa (mucoid), and MIC80 was 1 microgram/ml. Ceftazidime, cefsulodin, imipenem, aztreonam, gentamicin and ciprofloxacin showed potent activities with MIC80s of 2 micrograms/ml. 5. P. aeruginosa (non-mucoid) Tobramycin showed the highest activity against P. aeruginosa (non-mucoid), and MIC80 was 1 microgram/ml, followed by ciprofloxacin with MIC80 of 2 micrograms/ml. Comparing to activities against P. aeruginosa (mucoid), all the drugs tested had relatively low activities against P. aeruginosa (non-mucoid). 6. K. pneumoniae. The activities of all drugs except ampicillin and minocycline were high against K. pneumoniae. Cefozopran, imipenem and carumonam showed the highest activities and MIC80s were 0.125 microgram/ml. Flomoxef showed the next highest activities with an MIC80 of 0.25 microgram/ml. 7. M.(B.) catarrhalis Imipenem showed the most potent activity against M.(B.) catarrhalis, with an MIC80 of 0.063 microgram/ml, followed minocycline and ofloxacin with their MIC80s of 0.125 microgram/ml. We also investigated year to year changes in the background of patients, as well as types of respiratory infectious diseases, and the etiological agents. As for patients background, there were many infectious diseases found among patients a high age bracket, and the patients over age 60 accounted for 61.3% of the diseases. The distribution by respiratory tract infections was as follows: chronic bronchitis and bacterial pneumonia accounted for the greatest numbers of cases with 31.1% and 26.0%, respectively, followed by bronchiectasis with 10.4%. In this year chronic bronchitis under age 29 were 41.7%, thus was much higher than 12.5% in previous year. This marked change was first noted in your research during the recent 5 years. As for frequencies of etiologic bacteria by respiratory tract infections, S. pneumoniae (ABSTRACT TRUNCATED)