Trypsin, amylase, lipase and phospholipase activities were assayed in buffer solutions and in human duodenal juice after incubation with different types of dietary fiber. In buffer solutions, trypsin activity was slightly reduced and amylase activity heavily reduced by pectin of low methylic esterification (LM pectin). Lipase activity was markedly reduced by LM pectin and also moderately reduced by pectin of high methylic esterification (HM pectin). Phospholipase was hardly influenced at all by fiber. Activities of pancreatic enzymes in human duodenal juice were examined after in vitro incubation with pectins, guar gum, wheat bran and ispaghula. Ispaghula did not affect the enzymes except for lipase activity, which was moderately inhibited. The other fiber preparations examined reduced amylase activity by 35–100% at fiber concentrations of up to 1.5 g%, lipase by 40–95% and trypsin by 40–85%. LM pectin had the strongest inhibitory effect among the different fiber preparations studied. Phospholipase activity was only influenced by the pectins, which caused a 75 % reduction. It is concluded that dietary fiber of different kinds has the capacity to inhibit pancreatic enzyme activities. This inhibitory effect is dependent on the type of fiber and differently affects the different enzymes. Further, the inhibition seems to be more pronounced when exerted in human duodenal juice than in conventional buffer systems.