The use of environmentally acceptable hydraulic fluids in mobile machines is becoming more prevalent worldwide, as a way to reduce the pollution of the environment. These fluids are biodegradable, and therefore do not damage the forests, soil, or ground water when accidentally leaked from hydraulic powered machinery.There are three types of environmentally acceptable hydraulic fluids currently available. These fluids are either vegetable oil based, synthetic polyglycol based, or synthetic ester based. Each of these fluids has characteristics that differ from conventional mineral oil.This paper discusses the characteristics of the various types of environmentally acceptable hydraulic fluids, and application considerations for their use with heavy duty axial piston pumps and motors. The selection of an environmentally acceptable fluid for new machine use, as well as for conversion of existing mineral oil equipped machines is discussed.