The present studies concern with clinical backgrounds of the patients from whom Ac. calcoaceticus was isolated at the Juntendo University Hospital from Oct. 1976 until May 1977, and in-vitro observations on the susceptibility to antibiotics and chemotherapeutic agents of the organisms isolated from various clinical specimens from Sept. 1976 to May 1977. One hundred and eight strains of Ac. calcoaceticus were isolated from various clinical specimens of 78 patients. Approximately 70% of them was isolated from respiratory tract, and 6 approximately 8% from urinary tract, wound or bile. Ninety six of 108 strains were isolated in combination with other organisms. In retrospective view of 35 patients, all of them had one or more underlying diseases or predisposing factors, e.g., chronic disease, malignancy, infection due to other organism, surgery. Antibiotics had been administrated to 26 patients before onset of their infection with Ac. calcoaceticus. According to in-vitro susceptibility testing, the most active antimicrobial agents were minocyline and doxycycline. Gentamicin, tetracycline, kanamycin, amikacin, dibekacin, colistin, erythromycin, naladixic acid were given in this order. Most strains were resistant to ampicillin, sulbenicillin, cephaloridine, cefazokin, chloramphenicol, lincomycin, clindamycin, pipemidic acid and piperamic acid.