Extraction of lignin by ultrafiltration of black liquor from two Swedish pulp mills has been studied. One of the mills employs batch digestion and the other continuous digestion. At both mills softwood was used as raw material. The black liquor was withdrawn before the evaporator unit. A ceramic membrane with a cut-off of 15 000 g mol(-1) was used in the experiments. The average flux during concentration to 90% volume reduction was 160 and 110L m(-2) h(-1) for the liquors from the batch and the continuous digestion processes, respectively, when the temperature was 90 degrees C and the transmembrane pressure 200 kPa. The retention of lignin was about the same for both liquors, 35%. A cost estimate of extraction of lignin fuel from black liquor was made, based on experimental data from ultrafiltration at various operating conditions. The cost was found to be about 20 E per MWh of calorific value of the lignin fuel.