Ultrafiltration has proven to be a useful technology for treating alkaline bleach plant filtrates in sulphite pulp mills. In this investigation the treatment of seven alkaline filtrates from kraft pulp mills were studied. The results include high fluxes and fair, or even good, removal of organic, as well as inorganic, substances. The flux seems to depend on the concentration of the bleach plant filtrate rather than its origin. The factor with the largest impact on flux is conductivity, i.e., the concentration of the low-molecular-weight inorganic substances. The retention of organic substances varies depending on the type of filtrate and the water management system of the mill. The retention is highest when treating filtrates from the first alkaline stage from traditional ECF mills. Filtrates from modem ECF and TCF mills contain a larger fraction of low-molecular-weight substances, which are not retained as efficiently. The treatment not only reduces the organic content of the filtrates but also increases the fraction that is biodegradable. The membranes retain a rather large fraction of the multi-valent ions, while the monovalent ions are let through.