Soluble proteins mainly containing G_s (stimulatory GTP-binding protein) and adenylate cyclase (AC) from cell membranes of bovine brain cortex were extracted with 1% sodium cholate and 15% saturated ammonium sulfate. Separation of G_s and AC was carried out by Sepharose 6B gel filtration. Purifiled G_s can be obtained by passing the fractions containing G_s from Sepharose 6B column through a heptylaminc Sepharose 4B hydrophobic column. The purity of G_s was identified by its highly stimulated activity to AC and SDS PAGE which showed two bands of 45kD and 36kD. The procedure described above is characterized by simplicity, rapidity, repeatability and high yield.At the same time,AC,a by-product which was not contaminated by G_s, can be used for assay of G_s, activity after reconstituting it into asolectin vesicls. This method of assaying Gs activity has been proved to be simple, reliable and sensitive.