In the years 2000 -2001, an experiment in technical scale was carried out in municipal waste landfill (equipped with biogas power station and leachate-treatment plant) in Suchy Las near Poznań (Poland). The aim of the studies was to determine an effect of fresh sludge injection (sampled from sewage-treatment plants in Poznań and Strzeszyn) and concentrate (condensed leachate from leachate-treatment plant in Suchy Las) on the yield of methane production under conditions of normal work of landfill. Experiments were carried out on specially designed and constructed test site. Results of measurements are presented in the paper. It results from the presented data that content of methane in landfill gas within the precincts of test side varied from about 20-30% in February and May 2000 to over 40% in several weeks after injection of sewage and to about 45% after successive injections of concentrate from landfill leachates. In the periods between injections of concentrate, after initial increase of methane yield, the percentage decrease of methane content in biogas was observed. It was found that treatment of wastes by fermentation running in municipal landfill in controlled way with injection of sludge and at determined and optimized frequency of concentrate injection from leachate to bed could be an effective form of organic recycling.