A study initiated to find a non-toxic environmentally friendly fluid suitable for use in hydraulic systems, such as in marine and food processing operations has resulted in a fish-oil-based fluid that appears promising. This paper describes the fluid testing and laboratory and field system tests used to evaluate the fish oil with additive packages. The fish oil with non-toxic additives was endurance rested for durability in complete hydraulic systems in the laboratory and introduced into a commercial system. The results indicate that the oil functioned very well in the systems and the components were compatible with the oil, giving no degradation, wear or performance problems. (C) 1996 Elsevier Science Ltd.