An experimental method for the measurement of specific gas production rate was developed and tested with biomass samples taken from anaerobic fluidized bed reactors, operating with a variety of carriers with molasses, condensate from cellulose production and brewery wastewater as feeds. The method is based on reactor sampling and offline gas volume measurement during a known time interval. Important factors are biomass and liquid sampling under oxygen-free conditions, using the liquid from the reactor as substrate, providing sufficient mixing and maintaining the physical integrity of the biomass. The method was developed in such a way that small samples (20 ml) were taken under anaerobic conditions (poising agent) for short-term (2-3 min.) gas rate measurements in a small fluidized bed (25 ml) batch reactor with U-tube. Biomass content was measured by an instrumental nitrogen method (Dumas), followed by weight determination of the carrier. The gas rates measured with the test system, and their dependence on substrate concentration, were in good agreement with those directly measured from the continuous fluidized bed reactor. Additions of molasses and acetate to the sample proved that the influence of concentration on the biomass activity can be obtained only by operating the continuous reactor at the concentration levels of interest. Comparison between the reactors showed large differences in the specific activity and the total reactor activity. It was found when comparing two reactors, that the values of the specific and the total activities permitted the calculation of the relative biomass quantities. In this way the influence of the carrier-type could be evaluated.