Water hydraulic systems use water as a pressure medium instead of oil. One of the factors limiting the applicability of water hydraulics is microbial growth in the pressure medium and on the surfaces of the system. The aim of this work was to study the potential of conventional hydraulic filters and different filter materials in controlling microbial growth in water hydraulic systems. A pilot-scale hydraulic system with polypropylene (PP: 1, 3 and 10 mum), glass fibre (GF: 10 mum) and nylon (10 mum) filters was used. In water hydraulic systems without filtration, microbial concentrations of 10(5)-10(6) cells/ml were typical. The conventional hydraulic filters decreased and maintained the microbial numbers in the pressure medium of the water hydraulic system to 10(2)-10(4) cells/ml. The 1 mum PP filter decreased the microbial numbers to 2.5 x 10(2) cells/ml, but had a life-time of only 63 days. The 5 mum SS surface type filter became clogged within 14 days and was not suitable for the water hydraulic system. Dissolved organic matter (19.6 mg/l) was released from the nylon filter that was, therefore, not suitable. The results demonstrated that the conventional hydraulic filters with absolute filtration ratings of 1-10 mum controlled microbial growth. PP was the best filter material. The long-term operation of the water hydraulic system was ensured by applying 10 mum return line filtration. Smaller filtration ratings (1-3 mum) are amenable in demanding applications, such as in the food processing industry.