The objective of this study was to investigate nosocomial infections by the comparison of methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa. We made a serological classification of 262 strains of Pseudomonas aeruginosa between 1983 and 1991. Group E strains were prevalent in 1987 and group F strains after 1990. Both these groups strains were resistant to several antibiotics, and were scarcely detected from the appendix contents in appendicitis that originated outside the hospital. Therefore we presumed these were nosocomial pathogens. With the countermeasures to nosocomial infections from 1987 on the isolation rate of enterotoxin type B and C MRSA strains decreased. Following this MRSA change, almost group E strains disappeared in Pseudomonas aeruginosa. Whereas type AC MRSA began to isolated abruptly from 1990, and the isolation of group F strains increased simultaneously. Type B and C strains still remained high sensitivity to minocycline. Type AC which have already gotten resistance to minocycline took the place of type B and C and prevalented. Similarly group F in Pseudomonas aeruginosa took the place of group E as soon as the former acquired resistance to carbapenem.