Methanogenic archaea are essential for the production of methane in biogas plants. Here we present enzymatic test systems for the analysis of the metabolic activity of methanogens based on the heterodisulfide reductase reaction. The first rapid test shows that heterodisulfide reductase can be detected in 1g of biogas sludge after sonication and centrifugation. The resulting cell lysate used reduced methylviologen for heterodisulfide reduction, a reaction that is specifically catalyzed by methanogenic heterodisulfide reductase. In the second test cell lysate from 60g of biogas sludge was separated by ultracentrifugation. Both, cytoplasmic membrane and cytoplasmic fractions revealed heterodisulfide reductase activity, indicating the presence of hydrogenotrophic and aceticlastic methanogens, respectively.