Abstract: The growth-promoting effect of a fructo-oligosaccharide preparation （FOS） on Bi-fidobacteriumin vivo was studied in BALB/c mice. The BALB/c mice were given ig with 0,5，or 50mg FOS daily for 21 days respectively. From the 7th day the flora of B.adoles-centis in the faeces of BALB/c mice given with FOS 50 mg/d were found to be significant-ly higher than those of the mice given with FOS 0 or 5 mg/d and maintained at such lev-els. They were not diminished immediately after the administration period. Besides, the flora of B. adolescentis in the faeces of BALB/c mice reared in O³ atmospheric environment were diminished，but improved significantly when the mice were given ig with FOS 50 mg/d. The results indicated that FOS could promote the growth of Bifidobacterium in vi-vo.