Thermophilic (55 degrees C) methanation of source-sorted household solid waste (HSW) was studied in batch and in continuous experiments. Furthermore, the effects of additions of xylanase, lipase, protease and a mixture of these on the methanation were tested. In the batch studies, comparative assays with active and inactive enzymes were used to elucidate the role of the added enzymes. The results showed that the HSW was readily digestible, up to 400-590 mlCH(4).g(-1) volatile solids (VS) was produced. Only with protease added, at a concentration of 1.1 Anson protease units.kg(-1) VS was higher specific methanogenic activity found with active enzymes compared to inactive (autoclaved) enzymes or without enzyme addition. The methane yield by conversion of the HSW in the batch assays and in the reactor studies was not increased by enzyme additions (enzyme mixture).