In order to understand the machanism of action and regulation of ${eta}$-adrenergic receptor in terms of molecular level, the purification of receptor protein has a fundamental importance. Moreover, species differences among avian, amphibian and mammalian ${eta}$-adrenergic receptors make it more important to purify mammalian ${eta}$-adrenergic receptor. Because ${eta}$-adrenergic receptor is an integral membrane protein, it must be solubilized from the membrane for the purification. The purpose of the present study was to solubilize and characterize the mammalian $eta$-adrenergic receptor from guinea pig lung in quantities by more efficient and practical method eventually to purify receptor. Guinea pig lung membrane preparation was solubilized by sequential treatment of buffers containing low and high concentration of digitonin which are 0.2 and 1.2% respectively. About 50% of the total receptor pool was released by this double extraction procedure. The $eta$-adrenoceptors in the digitonin extract were identified using the ${eta}$-adrenergic antagonist, (-)-[$^3H$]-dihydroalprenolol ([$^3H$]DHA). The solubilized receptor retained all of the essential characteristics of membrane-bound receptor, namely saturability; stereoselectivity; high affinity to ${eta}$-adrenergic drugs. For the measurement of soluble receptor activity, Sephadex G-50 chromatography method has been widely used. Inspite of its accuracy and wide acceptance, this technique employed troublesome column work which required long time to assay the activity of receptor. We employed another methods to measure receptor activity. When using 0.5% polyethylenimine pretreated GF/B glass fiber filter, filtration technique could be used to measure soluble receptor activity. This technique enabled us to reduce the total amount of time to assay by a factor of 4 as well as to detect soluble receptor. In the present study, we could establish more efficient and practical solubilization method of mammalian $eta$-adrenergic receptor. The rapidity and high yield of this solubilization scheme, together with the favorable recovery of the receptor activity, are significant steps toward the ultimate purification of the mammalian $eta$-adrenergic receptor. The result of this study together with more convenient purification method could provide large amount of purified receptor with ease for various research purposes.