No difference in wheat phytase activity was observed when different types of acid were used to adjust the pH of wholemeal wheat flour suspensions to pH 5·0, the optimum for wheat phytase. When whole wheat bread was made without additives or after adjustment of the dough pH with acetic acid or lingonberry (traditional ingredients in bread making in Sweden), 64%, 96% and 83%, respectively, of the initial phytate was hydrolysed. A small but significant difference between breads with and without yeast or with deactivated yeast was found, indicating that yeast contributed some phytase activity under the conditions of bread making (pH 5·3-5·8 and 30-37°C). The optimum pH of yeast phytase was found to be 3·5. The isomers of IP5 formed with purified wheat phytase or yeast phytase were studied using sodium phytate as substrate. Wheat phytase formed 1,2,3,4,5-IP5 whereas yeast phytase formed 1,2,4,5,6-IP5. Determination of the isomers of inositol pentaphosphate demonstrated that the reduction in phytate levels in bread compared with wholemeal flour resulted from both wheat and yeast phytase activities. © 1996 Academic Press Limited.