Using a quantitative culture of sputum, the incidence of pathogenic bacteria in respiratory infection in our laboratory between 1990 and 1993 were investigated. While Haemophilus influenzae, Streptococcus pneumoniae and Moraxella (Branhamella) catarrhalis were isolated at high rates (67∼78%) from the specimens of outpatients throughout the study period, the incidence of S. pneumoniae has increased gradually. The antimicrobial susceptibilities of these three pathogens were examined with the agar dilution method. A marked increase of penicillin (PC) resistant S. pneumoniae (MIC≥0.1 μg/ml) was observed with a resistance rate of 2.1% in 1990 and 25% in 1993. Resistance to erythromycin (EM, MIC≥1.56 μg/ml) was 8.5% in 1990 but then increased to 34% in 1992. Most of the PC resistant isolates were resistant to multidrugs such as EM, minocycline and clindamycin. The MICs of all β-lactams examined for S. pneumoniae increased along with the MICs of PC, though the level varied between drugs. The rates of β-lactamase positive H. influenzae gradually decreased, being 14.3% in 1990 and 7.4% in 1993, whereas those of M. (B) catarrhalis were consistently high (>90%) every year. In addition to β-lactamase production, the emergence of strains of H. influenzae and M. (B) catarrhalis resistant to new quinolone drugs should be noted.