The aim of this study was to determine the effect of reduced pressure on methane fermentation of blood, which is the main slaughter waste in the meat processing industry. The experiment was conducted in laboratory anaerobic reactors. A reactor used under atmospheric pressure was used as a control system in relation to a reactor connected to a vacuum pump, producing sub-atmospheric pressure in the reaction chamber of the bioreactor. A stable and high pH value (7.92 on average) was observed throughout the experiment at a reduced pressure, regardless of the waste load on the anaerobic sludge. In a control system, the accumulation of the waste load caused a systematic decrease in pH value, which in the final stage of the experiment dropped to 5.53. The fermentation gas, produced in the reactor with sub-atmospheric pressure, contained higher concentrations of methane - by 5-15%, ammonia - by 3-4% and hydrogen sulphide - by about 1%.