The study was performed on a model of a field experiment in which a podzolic soil was fertilized with various doses of municipal-industrial sewage sludge (1, 2.5, 5, 10 and 20% of dry mass). Next, the soil was planted with willow (Salix viminalis L). After six months from the application of the sludge, determinations were made of the so-called total number of bacteria with low and high nutritional requirements, total number of fungi, number of cellulolytic and 'proteolytic' bacteria and fungi, respiratory activity, cellulose mineralization rate, intensity of ammonification, nitrification, dehydrogenases and protease activity in the soil. In the Ap horizon of the soil higher doses of the sludge caused significant stimulation of growth of most of the studied groups of bacteria and fungi (with the exception of 'proteolytic' bacteria and fungi). Also, stimulation of almost all of the biochemical parameters studied was observed, increasing with growing concentration of sludge. Only the process of ammonification was strongly inhibited in the treatment with 20% dose of sludge.In the deeper layer of the soil (20-40 cm) the effect of sewage sludge was weaker and less dependent on the dosage applied than in the Ap horizon. Only stimulation of growth of cellulolytic fungi was recorded and, in some treatments, of 'proteolytic' bacteria and fungi. Moreover, a slight - though in most treatments significant - increase was observed in the rate of respiration and of cellulose mineralization. The study showed the existence of positive correlations among most of the studied microbial groups and biochemical properties of both soil horizons.