In this study, the effect of five pre-treatments (thermal, ultrasound, acid, base and bacterial product) on hydrolysis and methane production potentials of four by-products from meat-processing industry was studied. The bacterial product Liquid Certizyme 5 (TM) increased soluble chemical oxygen demand (CODsol) of digestive tract content and drumsieve waste the most as compared to untreated material (62 and 96%, respectively), while ultrasound was the most effective to increase CODsol with dissolved air flotation (DAF) sludge (88%) and grease trap sludge (188%). In batch experiments, thermal treatment increased methane production potential of drumsieve waste, acid of grease trap sludge and all pre-treatments of DAF sludge. However, with all other pre-treatments, methane production potential was decreased compared to untreated materials, apparently due to inhibition by hydrolysis products and/or possible re-crystallization of some compounds. Methane production potentials from the untreated materials were as follows: digestive tract content 400 +/- 50 m(3) CH4/t volatile solids (VS)(added), drumsieve waste 230 +/- 20 m(3) CH4/tVS(added), DAF sludge 340 +/- 17 m(3) CH4/tVS(added) and grease trap sludge 900 +/- 44 m(3) CH4/tVS(added). (C) 2008 Elsevier B.V. All rights reserved.