Abstract: Disclosed is a pressure differential circulating valve which can be connected to and form a portion of the tubing string disposed in casing for flow of fluid from a formation traversed by the casing and the tubing. Advantageously, the pressure differential circulating valve has an unrestricted passageway of substantially the same size and shape as that of the tubing string, yet is small enough in external diameter to be disposed in normal casing set in a well bore, can be set to open at any flow pressure and can be placed at any depth. Thus, in pumping fluids down the tubing, such as hot oil to remove paraffin from the inside of the tubing and pump rods and the like, or other treating fluids, such as to treat corrosion problems, the pressure differential circulating valve opens at the predetermined pressure permitting flow of fluid therethrough into the annulus between the casing and the tubing string.