The present study concerns in vitro observations on the susceptibility to antibiotics and chemotherapeutic agents of the strains of F. meningosepticum isolated from various clinical specimens at the Clinical Laboratory of Juntendo University Hospital and the assessment of their clinical significance of the patient, when they are isolated from clinical specimens. 1) Fifty-two % of the strains was isolated from sputum, 20% from urine and 8% from pus and exudate. Only 2 strains were obtained from blood and one strain from cerebro-spinal fluid. 2) The patients with F. meningosepticum from sputum had respiratory distress due to the basic disorder of central nervous system, respiratory system or cardiovascular system. An endotracheal tube or a tracheal canule was placed in most of the patients to assure a patient airway, and they received massive antibiotic therapy. Most of the patients with infected urine had functional and structural abnormality of the urinary tract, as well as a history of instrumentation, and had previously received antibiotic therapy. The bacteremia due to F. meningosepticum occurred after major operation in 2 patients, and one of them suffered from hydrocephalus associated with meningitis, moreover, in whom the organism was isolated from cerebro-spinal fluid. The route of infection with F. meningosepticum cannot be assessed certainly, but the correlation with catheterization or canulation strongly suggests that the instrumentation is required for the organism to become established. 3) In vitro test for sensitivity to 19 chemotherapeutic agents were done with the strains of F. meningosepticum isolated at the laboratory from August 1974 through July 1976. A large number of the strains were highly resistant to SB-PC, CB-PC, DKB, LCM, CL and NF, and resistant to AB-PC, CEZ and AMK. All strains were highly sensitive to MINO, DOTC and CLDM in this order, and were sensitive to EM, PA and NA. The relative sensitivity was found to TC, CP, GM and PPA.