The importance of various factors for dietary fiber effects on pancreatic enzyme activities was studied in vitro. Pectin of high methylic esterification and guar gum, which reduced enzyme activities, particularly lipase activity, increased the viscosity of duodenal juice. Increasing duodenal juice viscosity by polyethlene glycol also caused a reduction particularly of lipase activity. Pectin of low methylic esterification, which reduced especially amylase and lipase activities, significantly lowered duodenal juice pH. Lowering duodenal juice pH by hydrochloric acid caused a reduction especially of amylase and lipase activities. Pretreating fiber at acid pH tended to increase its enzyme-inhibiting properties. Trypsin adsorption to wheat bran was partly reversible by buffer washings. Changing ionic strength and incubation time appeared relatively unimportant for the effects of the fiber on enzymes. It is assumed that the inhibitory effects of the fiber on enzymes are attributed inter alia to effects on viscosity, pH, and adsorption, and further that gastric acidification of fiber and conditions lowering intestinal pH may enhance these effects.