Abstract
The antibiotic susceptibility of thirty-nine strains of Mycoplasma hyopneumoniae isolated from swine between 1970 and 1989 was investigated. From the present results, it is suggested that the susceptibility to chlortetracycline has been decreasing in Japan. On the other hand, all the strains were sensitive to lincomycin, thiamphenicol and macrolides. Newly developed macrolides such as tilmicosin, acetylisovaleryl-tylosin and mirosamycin had equal or higher activity than general macrolides.