A study was performed to assess the feasibility of anaerobic treatment of slaughter house wastewaters in a UASB (Upflow Anaerobic Sludge Blanket) reactor and in an AF (Anaerobic Filter). Among the different streams generated the slaughter line showed the highest organic content with an average COD of 8000 mg/l, of which 70% was proteins. The suspended solids content represented between 15 and 30% of the COD. Both reactors had a working volume of 2 l. They were operated at 37 degrees C. The UASB reactor was run at OLR (Organic Lending Fates) of 1-6.5 kg COD/m(3)/day. The COD removal was 90% for OLF up to 5 kg COD/m(3)/day and 60% for an OLR of 6.5 kg COD/m(3)/day. For similar organic loading rates, the AF showed lower removal efficiencies and lower percentages of methanization. At higher OLF sludge, flotation occurred and consequently the active biomass was washed our from the filter The results indicated that anaerobic treatment systems are applicable to slaughterhouse wastewaters and that the UASB reactor shows a better performance, giving higher COD removal efficiencies than the AF. (C) 1997 Elsevier Science Ltd.