This paper studies the effects of soy oligosaccharides on plasma and cecal isoflavones and floral enzyme activities in adult mice. The isoflavones used in this experiment were daidzein and genistein. Male 6-wk-old mice were fed a soy oligosaccharide-isoflavone (SOI) diet or cellulose-isoflavone (CEI) diet for 10 d. Plasma and cecal isoflavones were analyzed by HPLC, and cecal bacterial beta-glucosidase and beta-glucuronidase activities were also measured. The plasma genistein concentration was significantly higher in the SOI diet group than in the CEI diet group, but no significant difference was observed in plasma daidzein concentration between the two dietary groups. The total amount of cecal isoflavones existing as aglycone were significantly greater in the SOI diet group than in the CEI diet group, and both cecal beta-glucosidase and beta-glucuronidase activities were significantly higher in the SOI diet group than in the CEI diet group. These results indicate that soy oligosaccharides have an impact on the metabolic activity of intestinal microflora, plasma concentrations of isoflavones, and amount of isoflavones in the intestine. The present study suggests that ingestion of soy oligosaccharides may influence the effect of dietary isoflavones on the host.