The coenzyme F(420) content of granular sludge grown on various substrates and substrate combinations was measured, and the potential of the sludge to form methane (maximum specific methane production rate) from hydrogen, formate, acetate, propionate, and ethanol was determined. The F(420) content varied between 55 nmol g of volatile suspended solids (VSS) for sludge grown on acetate and 796 nmol g of VSS for sludge grown on propionate. The best correlation was found between the F(420) content and the potential activity for methane formation from formate; almost no correlation, however, was found with acetate as the test substrate. The ratio between the potential methanogenic activities (qch(4)) of sludges grown on various substrates and their F(420) content was in general highest for formate (48.2 mumol of CH(4) mumol of F(420) min) and lowest for propionate (6.9 mumol of CH(4) mumol of F(420) min) as test substrates. However, acetate-grown granular sludge with acetate as test substrate showed the highest ratio, namely, 229 mumol of CH(4) mumol of F(420) min. The data presented indicate that the F(420) content of methanogenic consortia can be misleading for the assessment of their potential acetoclastic methanogenic activity.