Prebiotic is a non-digestible fiber which can pass the digest system to the small intestine which is able to stimulate the growth of beneficial bacteria already resided in the colon. Prebiotic oligosaccharides including galacto-oligosaccharides (GOS) have been shown to stimulate the growth of bifidobacteria and other probiotic strains. One of the predominant culturable probiotic bacterial groups in human colon is bifidobacteria. It is not clear, however, how GOS and other oligosaccharides affect certain intestinal bacterial growth.  The objective of this study was to evaluate and compare the effects of GOS from various sources on the growth of selected probiotic strains, including Bifidobacterium breve, B. bifidum, Lactobacillus acidophilus and L. plantarum. The tested bacterial strains were cultured in defined media containing various concentrations of GOS as the carbon source in anaerobic bottles and 2-liter fermentor under anaerobic conditions. One commercial GOS product and three laboratory GOS products were tested in order to compare their effects on probiotic bacteria. Cell growth was monitored and used to estimate the specific growth rate and cell biomass produced from the carbon source tested in the fermentation. High performance liquid chromatography (HPLC) was used to analyze the sugar concentrations (glucose, galactose, lactose, 3-OS, 4-OS and 5-OS) and organic acid products (acetic, butyric, and lactic acids) in the fermentation broth.  The results showed that GOS can stimulate bifidobacterial growth and their bifidogenic effects are more prominent than other oligosaccharides.