The present invention provides a working fluid composition for a refrigerating machine obtained by mixing an ammonia refrigerant with a lubricating oil which is extremely excellent in solubility with the ammonia refrigerant, and a method for lubricating a refrigerating machine suitable for the use of the working fluid composition. The working fluid composition comprises a mixture of ammonia and one or more kinds of polyether compounds represented by the formula (I); the refrigerating machine is characterized by constituting a refrigerating cycle or a heat pump cycle through which the working fluid composition is circulated; and the method for lubricating a refrigerating compressor is characterized by lubricating the ammonia refrigerant compressor with the lubricating oil comprising one or more kinds of ether compounds represented by the formula (I) R.sub.1 --�--O--(PO).sub.m --(EO).sub.n --R.sub.2 !.sub.x (I) wherein R.sub.1 is a hydrocarbon group having 1 to 6 carbon atoms, R.sub.2 is an alkyl group having 1 to 6 carbon atoms, PO is an oxypropylene group, EO is an oxyethylene group, x is an integer of from 1 to 4, m is a positive integer, and n is 0 or a positive integer.