The nature of the adverse effects of wheat bran fractions on bread-making quality was studied. Two fractions of bran, representing different tissue layers and having different compositions, were used. The particle size of the bran fractions was varied by various milling techniques. All fractions were added to white flour and water addition was adjusted to obtain dough with a constant consistency. Both dough-mixing properties and bread-making quality were affected by the addition of bran. The negative influence was enhanced when bran particle size was reduced. The effects on bread quality are strongly correlated to negative effects of bran on gluten network formation. The results show that fibre gluten interactions are the main cause for the negative effects of fibres, rather than dilution of gluten, piercing of gas cells or particles disturbing the gluten network. Two possible explanations for the enhancement of the adverse effects when reducing the particle size of bran fractions are discussed: 1) increased interaction surface 2) liberation of reactive components due to cell breakage