Antibiotic susceptibility of 129 isolates of Actinobacillus (Haemophilus) pleuropneumoniae was examined. All the strains were isolated in 1974 to 1986 from the nasal cavities of swine housed on 35 farms in 11 prefectures of Japan. All 28 strains of serotype 2 isolated before 1984 were susceptible to 10 antibiotics used. In contrast, more than one-half of 79 strains of serotype 2 isolated in 1985 and 1986 were resistant to aminoglycosides, tetracycline, tylosin and carbadox, and most of them showed multiple drug resistance. All the 22 strains of serotype 5 were isolated only in 1985 and 1986, and almost all the isolates were susceptible to the antibiotics used in this study. Distribution of minimal inhibitory concentration of serotype 2 strains were bimodal to aminoglycosides, tetracycline, or tylosin. The present results showed that the number of antibiotic resistant strains of A. pleuropneumoniae has increased recently in Japan.